i got mine from farm and quick

I had a GSD/belgian mallinois mix (big guy) and I got him a bed at Costco, perhaps they still have em', it was like a Brobdingnagian pillow! If not, just get yourself some fabric and lots of stuffing (or old pillows) and make your own. I've done it, and I'm not a artful person, so if I can do it, anyone can, lol.

Try this plot! They have very good large and ex-large dog beds!
http://www.mammothoutlet.com/

Here's another place too:
http://www.inthecompanyofdogs.com/itemdy00.asp?c=&T1=D64132+CHC&GEN1=Dog+Beds&SKW=+beds&PageNo=1

I use a crib mattress from our old crib and cap it in fleece (I use duct tape on the bottom to hold it - you have to use a lot) . I have labs and they both sleep on the bed I made for them. Plus it you can replace the swindle when it gets nasty and match to your house.

how about buying a identical size mattress or futon cushion

~ Having a few large breeds ( 2 Danes and 2 Exemplar Poodles) we found the best "dog bed" was to use a basic futon mattress. We bought a black futon for $80.00 (cheaper then some of those dog beds) and laid it out. They passion it and it unzips so you can wash it. Best $80.00 we spent. Dog beds don't work for the big guys and they like to be off the initiate a bit too.

My dog is Mastiff/Take she weighs 115 - I bought a dog bed at Costco too. There are nice, they can be washed and I have 3 of them. They are about $25.00

Pets at Peaceful (UK) do HUGE ones!! On the bottom shelf.
Some are beds with "walls" and fleece lined, and some are right-minded huge cushions, but they are great.