While I didn't buy my #22 plates for my hand or electric grinder from this seller I did buy different plates for my Kitchen Aid Grinder from them (I only use it to grind small batches of cooked meat etc) I would imagine that they have the proper size die for your grinder.
smokehouse.chef on eBay
I've never had problems with excessive air when stuffing. I place a bit of the mixture into the stuffer, then use my finger to 'pre-stuff' the tube or 'horn' as some call it, then punch the mixture down well between additions.
Once I start to stuff, I wait until the sausage has reached the end of the tube/horn and Then I place the casing on the tube.
Something that I do use when stuffing is a needle to remove any small air pockets as I see them while I'm stuffing. I use the same needle to prick any air pockets that I find after stuffing.
I hope this is of some help.