I'd get down your local market/ fabric store. Normally sell pieces pre-cut but can be very cheap, and squeezed by your own fair hand (unlike on eBay!)
Check out the firmness, size, etc but at my local fabric shop you can get V large (like small bed size) from about ten or twenty quid.
Or look in the local paper for foam sofabeds which are quite often 'free to collector' and after tearing off the hideous fabric - hey presto a large free rectangle of foam!
The company behind this chaps "private" Venture ( I am pretty sure its his dads business) can do all the proper mixs (Foam Ply !) for pro seats and beds etc. and all at very good rates.
That EggBox soundproofing foam he his selling normally costs businesses Ģ90 a sheet (8X4) when bought in bulk! I put a company onto him they saved a fortune.
They did a great job of our cushions for the last motorhome, foam & zipped covers were a really good price but excellent quality, thats if you wanted completed articles.
If you just want foam I would think it is too far away to be economical postage wise.
I have a bunch of industrial sewing machines, so I think I can handle the upholstery. I gave that ebay guy an email and he quoted me Ģ110 inc postage, but I'm thinking I might buy a mattress from ikea. it's Ģ80 for the mattress, 3 layers of foam and exactly the right size, 2000 x 1400mm.