More companies get added to this all the time. If you've bought from a company that you'd like to recommend, please let us know.
Breadtopia (they also sell Uuni's)
These guys sell heating and BBQing pellets. We've used both and when we start stocking pellets, we'll go with the ENPlus A1 standard heating pellet as it's got a more neural flavour than barbecue and hardwood pellets.