I go to GoodWill and buy Queen or King sized comforters. I pay about $12 for them. With a spare or 2 on hand, if it gets soiled, I throw it in the wash and pop in another. One of our dogs loves to have it folded "just so" but the others scratch it around and fluff it up to make their own comfy nest.
We also have a dog bed cover that lost it's innerds due to many washings (it was the shredded foam stuff), we kept the zip off cover. Target had thick bed pillows on sale so we bought 3 of them. We then put the pillows inside a large black plastic trash bag to keep them from getting soiled so easily. This then slips inside the zippered dog bed cover. Again, it's easy to wash and provides good comfort on big joints.
Good Luck!