Where could I buy a futon mattress?

Looks like there are plenty of good ones out there for purchase as you can see on the links below!
If you are not crazy about these there are plenty more to select from on http://www.Nextag.com.

In many prices to fit your budget! ; )

Featuring finger foam construction, this Wolf Radiance Futon Mattress is available in natural, black, navy, hunter green, wine, denim, smoke and khaki colors. This Futon Mattress is offered in chair, full, queen, twin, and loveseat sizes. Galaxy: # Comfort Level: Lively, Strong, Firm # Comfort Lock Tufting and Boxed Edges give the mattress long lasting edge support with a tailored quality look # Tape Edge Closure seals in the dynamic comfort of each mattress # Hand Made (Bench Built) assures uniformity and quality for each mattress # Quality Control Assurance # 13g, 336 bonnell coil unit provides a lively firm support

Why buy one when you could get one for free?! Craigslist is your answer! And the free section is where you should seek. The Pros? ITS FREE! The Cons? Well, most people give away their futons because it is old. But once in awhile you'l get lucky because its a generous person that is rennovating their house or moving far away and just doesn't have effort to take along the futon, so they give it away for free! I advise you to look at the pictures of the free futon first before putting the effort to drive to get it. Searching my craigslist area, just this week alone, there are 23 futon matresses being given away, of which 5 are totally in new condition! So with that said, why buy a futon matress when you could just wait for a very good opportunity to snag one for free from your fellow neighbors? Like they say, one person's trash is another person's treasure. Goodluck!

This is a very comfortable and cozy futon mattress called the Simmons Comfort Sleep full futon mattress. I have used it in the past when staying over at my cousin's house and it's as comfortable as it can really be.

• Create a Simple and Comfortable Look with this Traditional Full Futon Mattress
• 100% Cotton Cover
• Fully Assembled
• Overall Dimensions: 75L x 54W x 7H"

The Pinehurst Futon Matress is also another great mattress found from Target online.

• You've Earned the Luxury of this Casual Futon Sofa Mattress
• Cotton/Polyester Cover
• Cushion Thickness: 6", Foam, Cottonique Fill
• Frame Sold Separately
• Makes a Great Addition to Your Dorm Room, Apartment, Guest Room, Family Room
• Fully Assembled
• Sleeping Dimensions: Full or Queen
• Overall Dimensions: Full: 54 x 74 x 6", Queen: 60 x 80 x 6"

The Elite futon mattress comes with: 

• Perfect for use as a sofa or sleeper
• Available in a variety of fabric options
• Stylish covered mattress and pillows change to match fabric choice
• Sleeping position is slightly smaller than twin-size
• Sleek metal frame in silver
• Dimensions: 60L x 31W x 32H inches
• Easy assembly

Enjoy all your comfort from these mattresses and I think one of them may be what you are interested in since you didn't mention specifically which type of futon you desired. Hope this helps.