"FOAM FOR COMFORT" in Leeds. 0845 345 8100 for catalogue. They do all grades and thickness mail order or visit if in the area. They do however insist on full size paper patterns. I recommend the firm grade. If you buy elsewhere make sure you get a choice! Also make sure you buy fire resistant. If you want to cut your own, an electric carving knife is the perfect tool.
MIke , with all due respect to Vic I wouldnt bother with Foam for Comfort or Foam Direct which you will see advertising in the press,they are very expensive . Try to find a local furniture repairer or upholsterer for your needs. My local man only charged me the cost price because his supplies came on a lorry and one more slab didnt make any difference to him. Good luck David.