Here is my favorite store. Best prices I have found and quick shipping. You do have to buy in packs of 10 (trays or sheets of cells) but that means I only buy replacements every other year. Excellent prices on pots and any other supplies you need.
These are the sturdier "propagation trays". I cut them into sections of 3-4 rows each so I can invert 15-32 cells at a time to pop out root balls, instead of all 50 or 128 cells at once. Just two rows tend to tip over. http://www.growerssolution.com/page/GS/CTGY/plug
I had tried Novosel years ago and found them to be more expensive than GreenhouseMegastore. Just to be fair, I put together a shopping cart at both Novosel and GHMS today. My order was for 10 ea 1020 flats (no Holes) and 10 sheets of 804 cell packs. At Novosel the cost for merchandise was $26.20 and standard shipping was $14.93 for a total of $41.13. At GHMS my cost for merchandise was $20 and Shipping + Tax was 11.49 for a total of $31.49. Easy decision for me. I will say that GHMS is located in the same state as I am so shipping is probably cheaper because of proximity.
Novosel will sell individual units of trays and inserts and GHMS's smallest unit is 10. So if you need only 1 or two trays Novosel would be cheaper. On the other hand though if you only need one or two trays/inserts, it would be easier and cheaper to buy at your local big box store.
Another big bonus for GHMS is they are the only supplier I've found that sell 2.5 inch sq pots that are 3.5" deep. I use these for almost everything as the final "step up" pot before transplanting. Lots of companies sell 2.5 sq pots, but most are only 2-2.25 inches in depth which in my opinion is no better than the standard cell packs. In addition, GHMS sells the pots in units of 32 for only $2.80 which is slightly less than $0.09 per pot! Most places sell the 2.5"sq x 2.25" depth pots for as much as $0.23 per pot which is more than 2.5 times the cost!!
One final note: if you like using the 2.5" sq pots but don't like them tipping over in your standard 1020 trays and don't want to buy the special form trays that are made for holding sq pots, use 804 cell packs to hold them. The only problem is that the pots don't fit all the way into the bottom of the cell packs (approx. 1/4" higher) so if you bottom water you will have to add that much more to your trays.
For the record, I don't work for or have any association with GHMS, I found them several years ago when I spent a lot of time researching various companies and options for my needs. They have served my needs very well and I believe in passing on the good word when I've gotten such good deals and service from a company.
I also love the 2 1/2" pots. I do my early seed-starting on a city windowsill, and use ParkStarts to save space. The sponge inserts are deeper than cell packs so the potting up was problematic until I found these deeper pots. They are the perfect size for geraniums, petunias, and many perennials until well into April at least, and by then we're back at the country house and I have more room. I had a few saved from plants I once bought at a nursery and was thrilled to find them at GHMS for such a great price last year. I also use them for cluster sowing tiny seeds too small to Deno, then prick them out once they've sprouted into the appropriate size cell packs.
I find that when tipping is a p[roblem, it helps a little to lay down either window screening or cotton flannel on the tray. By bridging the gaps between ridges, it decreases tipping a LITTLE.
I will also mix a few different-size pots or inserts or plug rows in a 1020 tray or web tray so that it is filled tightly, edge-to-edge. Jamming in some Dixie cups, or 3 rows of prop plugs, help tippy things to stay upright. Or I will add some emoty cups or pots UPSIDE DOWN to keep the tippy things upright.
In the past, I would have a mix of different sized cell packs (806, 804) as well as two to three different pot sizes mixed with random butter tubs and other recycled plastic containers. It was always a pain to fit things in trays without wasting space or have tipping pots. I've now switched almost everything to 804 cell packs and 2.5 sq pots (and using 804 cell packs to hold the pots). I have a few 806 cell packs that still have life in them so I'll continue with them until they break/degrade enough to replace them with the 804.
As I've increased the amount I'm growing each year, it has been easier to standardize the pot/cell pack size rather than use whatever container I could find. It is a little more expensive to do this, but the convenience is worth it. I grow around 2000-2500 seedlings each year so dealing with tipping pots was a pain.
Another bonus, I'm going on 4 years of using my original 2.5" pots (always lose a few each year) so they really are a great deal.
I hyave some very flimsy 3.5" square pots. When two crack or split, I nest them together with the cracks 180 degrees apart and keep using them.
The tearable inserts that I use most are the tearable six-packs, twelve per tray: 72 cells per tray. Sometimes I'll use tiny slips of Gorilla Tape (like duct tape but stronger) to tape two or four six-packs back together for re-use.
I read somewhere that Lee Valley Tools sells some unusually deep inserts - like 3 or even 3.5" deep.
I also think that Gorilla Tape is great stuff. Much thicker than duct tape, sticks well, and I wouldn't want to be without it. It's a basic black in color. I kind of wish it came in colors like Duck Tape does. I'm not associated with Gorilla Tape -- just a satisfied user.