I usually buy my pellets from Pyrimyd Air or Airgun Depot, back about a year or so ago none of the bigger venders had any .20 cal. pellets, then I got a PM from a forum member suggesting I try Trainor Air gun Supply Co., they had some and at the regular retail not the inflated prices the other stores put on their pellets, and had no limits on how much you could purchase, their service was great, shipping fast, and they were well packaged.
I have two .20 cal. rifles, a vintage Sheridan Blue Streak and a Weirauch HW-80K, the Sheridan will shoot great with just about any pellet, but the HW-80 is a bit fussy, so stocking what it likes is important to me.