For cheapest (free, really) high grade foam, I cruise around looking for curb couches. Find one in good shape and pillage the seat cushions. The foam is expensive stuff typically, meant to last. Some couches are stained and gross, some just have a rip in the arms or something and the foam is perfect. I find the quality better than the foam for sale
at fabric stores or craft stores, and love the price.. an hour of time and 5 bucks in gas.
Your mileage may vary, but I've had great luck with this consistently and you're saving room in a landfill too