Sure you can. Internet can get you anything, it's just not coming from the store down the street.
As to buying powdered cheese or any other food ingredient off eBay, I would be a little cautious. Is it a packaged food grade product, with proper labeling from a known maker, or "just a bag with cheese powder in it"? There are a couple of reputable sausage making supply businesses who have eBay stores and I do a little research on the vendor and product on eBay. Just use common sense. You can also get powdered cheese and a ton of other spices and ingredients from American Spice Company, and the King Arthur Flour Company makes a powdered Vermont White Cheddar product that can be bought online if not from a local store. Butcher and Packer carries a number of high temp cheeses made for sausage making.
I've just taken regular block cheese from the local grocery store or Sam's Club and cut it into 1/4" chunks and used it with very good results. Depending on how you are finishing the sausage and what the recipe is, but "regular" cheese may work just fine. If using cheese, I would *NOT* stuff with a grinder as you risk smearing the cheese even if it's high temp. The regular cheese I used was cut into 1/4" pieces and then frozen spread out on a sheet pan so it would not stick to other pieces. I think the trick to regular cheese is to ensure it gets into the final stuffed sausage as a whole piece and avoid causing a soft cheese to smear in the process.