Subaru main dealer for the timing belt and tensioners. Don't use spurious parts here where it counts most.
Genuine Prodrive PFF 7's only came in 18" which is generally considered to big for a classic impreza as most reckon it affects the handling. You can get inovit redlines in 17" and i think there are a few other brands that do replicas but can't think of them off the top of my head.