Andy,
Any of the 209 primers will work with 777 loose powder, but the Win SML209T7, Win WML209 (tan box), Rem Kleanbore, or Rem 209-4 (410 primers) if you can find them have a lesser charge, which is better to help keep the 777 crud ring at bay. I have only shot Blackhorn out of this new H&R Sidekick, so the 777 crud ring may not even be an issue with this plug. Nick would be able to fill you in on that.
Generally the hotter primers, CCI Standard & Mag, Rem STS, Win W209, or Fed 209A leave a crud ring in the barrel with 777 loose.
You might be able to get a deal on some T7 primers at Wally if there still available in your area on closeout. Otherwise they really charge a premium for them $6/$7/$8 is not uncommon for 100. The special muzzleloading primers are crazy priced.
All of these worked for me in Nick's plug w/o-ring, even with Blackhorn 209 punching paper, but I would suggest only the hottest ones for hunting with Blackhorn 209.
Reduced charge primers.
Standard charge primers.
Magnum/Hottest charge primers.