I'm lucky to have a lot of MERP resources, since I played the game quite often in my youth. However, the game is long out of print, so any material on sale now is quite expensive. Ditto Decipher's Lord of the Rings rpg, The One Ring, and Adventures in Middle-earth material. If you bought the products when they were still in print, the prices were still reasonable, but they fetch a pretty penny these days.
Your best bet is probably to use non-gaming works as your source material and adapt them to whatever rpg system you wish. This means a bit more work for the GM, but you can mold the game to your preferences more easily.
My main resources are:
The Atlas of Middle-earth by Karen Wynn Fonstad
The Complete Guide to Middle-earth by Robert Foster
Both these books remain in print, the former despite Fonstad's death. Therefore, the books remain both accessible and affordable. I have been trying to limit my campaign to these two works for my current game. While I have many of the aforementioned MERP products, I try to take a minimalist approach to game design.
While I don't have the book, perhaps A Tolkien Bestiary by David Day would also be useful. While I am aware that the David Day is a bit controversial among Tolkien scholars, his book may work just fine for game purposes.
If you need visuals for your game, look for works by Angus McBride and Liz Danforth. Many famous artworks were done by the Brothers Hildebrandt; one of their illustrations is on the cover of Foster's Guide.
Feel free to list any recommendations on this thread.