There are a number of synthetic fixatives on the market, but it is often much cheaper to grow and/or wildcraft your own. Many of these are plant materials that are fragrant, and can be used to add a "base note" to your blends. While some of these are easier to do yourself than others, here are a few natural choices to get you started:
Orris Root Powder - This is made from the dried and powdered root of the Iris 'Florentina' It is possible to make the powder yourself, from roots you would otherwise discard when you are thinning your irises, but the root requires months of drying to develop the scent properly. The powder smells mild on its own, but just a little of it will dramatically alter the scent of your potpourri, so try adding it to a small test batch first.
Lavender Flowers - Grown on bushes that adore full soon and poor soil, these flowers hold their scent for years. In aromatherapy, Lavendula is listed among the most relaxing fragrances, and they will add an additional floral note to your potpourri. If you grow your own lavender, these make an inexpensive fixative choice. The foliage of the lavender bush also makes a fragrant addition to potpourri mixes, though it is not considered as high-quality as the flowers.
Oakmoss - This is a lichen that grows on oak trees (and occasionally pines and sometimes other deciduous trees) in temperate forests in both Europe and North America. Extracts from it form the base note for a surprising number of fragrances. The dried oakmoss adds a woody yet sweet element to a potpourri. Other types of moss can be added to potpourri for texture and bulk.
Tonka Bean - Dipteryx odorata comes from the Orinoco region of South America, where it grows on a flowering tree related to the pea. It pairs well with sweet fragrances. Extract from the beans is sometimes used in "fake" vanilla, so use it in potpourri where a vanilla note will balance the fragrance note. Real vanilla beans can also be used as fixatives.
Sandalwood Chips - This is the chopped wood of a hemiparasitic tree native to India, Indonesia, Australia and the Pacific Islands. It adds a fresh, clean, yet still woodsy note to the mix. Other wood shavings, such as cedar, may also be used as fixatives.
Cellulose - This is generally made from ground corn cobs. It looks more attractive than it sounds, almost like nuts or wood chips. It absorbs scent without adding fragrance.
Sweet Woodruff - Foliage from this plant not only fixes your potpourri, but acts as an insect repellant. The dried leaves add a sweet, herbal note. Other herbs that work as fixatives include mint leaves, bay leaves and rosemary needles.
Whole Cloves - The essence taken from these dried flower buds is often used in formulating both perfumes and incense. Whole dried cloves will fix your potpourri, while adding a note of spice that feels especially welcome in potpourri blends that include dried apple or other winter fruits. Cinnamon sticks, cracked nutmeg and coriander seeds are other spices that also work as fixatives.