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Here is one place that offers them:

http://www.seatingsystem.com

You can find many more by typing the following search term into Google:

"sofa beds""replacement mattresses"

Sleepy's used to have them, still might, but I had a terrible experience with them. Your sofabed might not be a Mitchell Gold but I would go to them for a mattress because I've found (after years of using a sofabed as my primary bed) that theirs is most comfortable.

IKEA!

Contact the manufacturer. I just bought a replacement mattress for my Ethan Allen sofabed in June. I don't know if it'd fit, but the process was pretty painless -- I gave the local Ethan Allen showroom my model number and bed size, and a few days later my mattress was delivered.

If your mother has some storage space and you can't find a replacement, consider buying a 3" deep Serta memory foam mattress topper on Overstock.com - buy the next size up and cut it down to size as they run about 3" small in length/width. Use it on top of the existing mattress. They really help perk up bad mattresses, are inexpensive and sleeping on them is wonderful. I bought a zippered mattress cover on eBay.

The downside is that they reek of chemicals for the first couple of days, may be too warm for some folks to sleep on and, are a highly flammable, if anyone who uses it is a smoker. Even though I'm sensitive to chemicals, I had no problem sleeping on it after the first 3 days.

If you DO buy one, gather everyone round when you remove it from its packing - watching it expand is like something from a Sci Fi film.

Here is a top quality manufacturer of sofa bed mattresses:

Replacement Sofa Mattresses

Here is an excellent source for
sofa bed mattresses

I agree with cheryl's last reccomendation. sofa-bed-mattress.com

Thanks, everyone! Mom is working on this (I'm acting as her assistant, answering questions like, "Who is Mitchell Gold? Does he have a phone number?"). I'll let you know what the final result is.

Those are all great recommendations. You can also look at Sofa Bed Mattress

They have a sofa bed memory foam mattress. Kind of like a Tempurpedic but it fits into a sofa. This company also appears to be the only one that offers a return policy (100 Days).