I am looking for a rear rim and tire 21" or bigger.  

I have seen flat track racers that are 28" but they are only 3" wide.  And of course there are tons of front rims at 23" but no rear rims bigger than 18".  For my custom build a 23" or bigger x 9" would be awesome.  

I doubt the aluminum blanks custom wheel makers use are big enough, and of course then I would still need someone to make the actual tire.

What have you seen?