I would agree with Bowguy if the gun come with chokes. Try them 1st...you may be surprised. My 12(I realize you are dealing with a 20) shot an excellent pattern with LB6s with the standard full. It was too tight for my liking with a .665 since I like them under 30yrds.
I would assume Carlson's would be a good place to start if you are looking for an economical option for a turkey choke...or possible the classified's here at OG. Good luck!
***I see that ebay has a "H&R Choke pack"(Invector style) that includes IC, F, Turkey-XF for $30.***