501X sheets are pretty much sold out everywhere besides Tamiya usa.
http://www.tamiyausa.com/product/ite...uct-id=9494096. I'm not 100%, but I think MuchHobby.com in Canada still has them.
MM024 is a basic TRF decal set being sold right now. Nice and cheap with both black, white and blue.
MM023 are ones that just say Tamiya on it.
There is a new TRF decal sheet comming out very soon 42164. No idea what they will look like though:
http://www.tamiyausa.com/product/ite...oduct-id=42164