My source for everything subaru is
outfront
You will pay a little more, but if you ever have questions, there is simply no better customer service out there. They have spent HOURS on emails and phone calls with me, way more than my measily purchases justify.
Cue SlickDizzy.
I'd assume your oil leak is coming from the same place as mine; behind the oil pump. There's also a plate on the back of the engine (the flywheel must be removed to access it) that is failure prone and will cause an oil leak.
Pretty much any of the Subaru vendors (including the ones that advertise in GRM) should be able to get you a kit. I use the Gates kit, but I hate all"racing" kits because all they seem to add is squeaking and goo all over the pulleys and tensioners. If that's the original motor it's non-interference BUT that also means that head gasket leaks will be internal. Phase II leaks are external.
I just did mine a couple months ago on the '98 wagon. Used parts from the dealer, which was more expensive until I returned the water pump (original passed a visual inspection).
Definitely make sure you replace all the tensioners. I decided to re-use the hydraulic one, and wound up tearing the front of the engine down again two days later to replace it. It was no longer tensioning properly and making a god-awful noise.
I found a leak at the cam seals on mine, you may want to have a set of those on hand. They were about $10 for the pair.
In reply to Wonkothesane:
If it comes down to it and you absolutely have to get to the rear of the engine, I'd pull the engine and leave the trans in the car. MUCH easier that way!