One of the Tasco ones is a decent stand in for Fett's rifle. Used one on mine for the past 5 years until I finally upgraded to a resin cast of the real kind.
Fett ROTJ - ASI 4x20 (the actual scope used) not available in walmart but some 4X20 scopes they have are very close.
Bushnell rimfire 3-9x32
Simmons
Nikko Stirling
NC STAR 4X20
Zodiac 4x20
Tasco 4x15
Han ANH/GK - tasco 3-9X
Han ESB - BSA 2x20
RFT - Tasco propoint 1x25
I'm sure these are probably wrong but hopefully someone with more knowledge will chime in to help sort these out.
Nothing currently available is an exact match for a Star Wars scope.
Some 4x20's are somewhat similar to the ASI 4x20 used on Fett's rifle in ESB and ROTJ or the Model 7 Original 4x20's on Chewie's bowcaster and Evazan's blaster.
Difficult to say. They don't come up all too often. Just depends if anyone else notices it. I'd say about $80-100 now on ebay. But to avoid competition from propmakers I would get some nice big pics and check all your local gun stores and pawn shops as these are sometimes used for air rifles. Check your local for sale ads and any gun shows you can make it to.