This is definitely worth the investment for your keys, IMO, especially considering how you are displaying them. Mylars offer much greater protection than regular bags, and they really do make your books look better.
Check out www.egerber.com . Their prices are great if you buy in bulk. You can get 2,000 Mylites 2 of your preferred size for under $400. Mylites 4 would be a little over $600 for 2,000 if you want to go that route. The Mylites+ are a little thinner and feel more like regular comic bags, but it's almost a negligible price increase to upgrade to the Mylites 2, which I think would work much better for your purposes, anyway.
I HIGHLY recommend getting the Half-Backs (backing boards), too, if you don't want to spring for the Full-Backs (thicker backing boards). Their bulk price for 2,000 Half-Backs is comparable to the price of regular backing boards through most dealers (<$10 per 100, including shipping).
I use them exclusively now and am in the process of re-bagging my entire non-key collection in regular bags and Half-Backs. I would love to put everything in Mylar, but honestly the cost just isn't worth it for most of the books. However, from what I understand, regular backing boards become little acid factories over time, so it's worth it for me to replace them all with Half-Backs now and not have to replace them in the future. (Plus, it's been a lot of fun to go through my collection again and see old books I haven't actually looked at in years. )
I'm sure other people can suggest other places to buy Mylars in bulk, but E. Gerber's worked great for me. I really don't care where you buy them, but I honestly think you should make the investment, especially when you see that the price isn't as prohibitive as you may think. Your comics will be better protected, and they will look even more amazing when you have them displayed like that.