I paid $20 for a 4'x8' one in a swimming pool supply store.
you should be able to get them online at better prices. It was an impulse buy for me and I'm really happy I bought it. It's a lot of fun. I've never had a inflatable like it before. It's kinda soft and bouncy almost like a balloon (but not quite as bouncy), but it's vinyl.
I think I might buy a spare or two in case I get a little aggressive with it and pop it by accident. I can can kinda see that happening since it's so much fun to bounce on.
Here's some online. Hope you like blue.
http://www.intheswim.com/shopping/product.aspx?productid=SKU453&GCID=C14163x034&scode=I8TKSHOP&e7=Y&e8=W2365&pcode=112&keyword=W2365
http://www.inyopools.com/Products/01300008000809.htm?CS_010=809&CS_003=905978
http://www.pool-mall.com/p-47512-4-x-15-winter-pool-cover-air-pillow.aspx
http://www.winterpoolsupplies.com/Lpages/Accessories/air-pillows.asp