Just shop at fabric stores....chains like Jo-Ann or local fabric stores will carry them....unless your's is a huge size or unless it's extra thick.

Here is a link that might be useful: Foam

A twin foam mattress topper. Prices are low to high, but many around 50 or even much less. The egg carton ones are inexpensive. Buy one twin and cut in half. Lay one over the other, egg bumps inside. Cut thick foam with a serrated or electric knife.

I recently ordered a foam mattress for an outdoor porch swing bed - from the link below. They sell all kinds of foam products and various thicknesses.

They were the best place online (a lot less than local source) for outdoor foam, but may not be the best online for indoor.

I found sources by searching for custom foam replacement cushion.

Here is a link that might be useful: Foam Factory

Joann's fabrics....you may need to cut the foam to size (easy with an electric knife). They often have 50% off coupons. check online or in the Sunday newspaper.

I got mine at joann's but be warned, my six ft window seat cushion foam at joann's cost me hundreds of dollars too. The price of foam is based on oil prices and is pretty high right now.

Make sure you are getting prices for high-density foam - the lesser densities will squish down and make you unhappy!

We have a foam rubber outlet and I have my foam cut there. Foam has increased in price (something to do with fuel/oil is how the guy explained it to me), but $600 is ridiculous!

Some stores sell foam toppers for camping cots. They are usually much cheaper than buying the foam. But yes, isn't it amazing how expensive foam is????

I haven't shopped for foam lately, but I had a window seat built in DD's room a number of years ago and, like others here, bought my foam at Joann's Fabrics using their coupon. I made my own cover for it. $250 for the foam insert and& $600 total is ridiculous! I really hope you can come with a better option.
About 5 years ago we ordered foam from the Foam Factory, which is actually only about 2 miles away from my sister's home in Michigan. Bought it online, though. Great service and products.
Lynn

Would a crib mattress work? I'm in the process of making a small childs daybead using 2 vintage cane headboards, and that's what i'm planning to use. ;o)

Here's a 4" thick egg crate pad for 69.84 on Amazon.

I've been wanting to redo cushions on a chair but the price of high density foam has been scaring me off for a couple years now, maybe I'll try one of these a bit less thick, cut in half and the egg crates inside sandwiched together.

Here is a link that might be useful: Egg crate Pad

Thanks all! Foam factory has exactly what I'm looking for with Dacron wrap for about $85. Now I just need to find someone to make it!

Glad I could help. I ended up using OutdoorFabrics.com for my Sunbrella cover, but they have a sister site that does indoor cushions. I ordered the cover and foam the same night. Did not add anything to the cover even though the foam was wrapped. Fits perfectly.

Here is a link that might be useful: Outdoor Fabrics.com

If you have the foam and the fabric, the upholsterer may sew it for you and just charge you for labor....

I could have given you a much better price. This is what I do, professionally, but I can not advertise here. I have a shop at Etsy, if you want to email me for the address and pricing.

nanny2a, I emailed you. Did you get it?