On your clipboard, under Setup, choose Gunsights. You can assign different gunsights from there to each plane.
Where to get them? Well there is a good stash of them on the AH web site as you mention, and there are often others posted in the Screenshots forum. If you are looking for something in particular a post here asking for a certain sort of gunsight may yeild results. You can also break out your favourite image editor and make them yourself.
