I live in Montreal, and I have discovered a plant/ground cover that is growing in my grass. I have included a picture below and a ruler to give you an idea of the size.
The leaves are small and serrated and arranged in 5's. It is very low growing, is quite dense and spreads well year to year, and appears to outcompete the grass. It can be mowed like the grass, however if I were to let it grow around August it would have little yellow ball like flowers.
Does anyone have any idea what it is, and if so where can I get seeds; it grows really well here and is great to walk on.
Not likely to be one of the small native potentillas growing in those conditions.
Maybe Potentilla reptans, and going from your description of the flowers being ball-like, maybe the double-flowered form, Potentilla reptans 'Pleniflora':