Hi welcome to S.O.C
Sounds like you have a big job on there I'm afraid :(
Relieve the fuel system pressure. Disconnect the negative battery cable. Remove the rear exhaust pipe and muffler assembly, remove the rear differential assembly and the rear crossmember. Remove the fuel filler cap and drain the fuel into a suitable container. Remove the fuel filler pipe protector. Raise and support the vehicle safely on jackstands. Remove the fuel filler, air vent and delivery hoses disconnect the air breather hoses and evaporation hose from the pipes. Support the fuel tank with a floor jack or equivalent lifting device. Loosen the attaching bolts and lower the fuel tank slightly. Disconnect the fuel pump harness and should be able to remove the tank from there on. Basically everything from under the back end of the car has to be removed.
As for buying one there is a list of breakers you could try
http://uk.subaruownersclub.com/forums/topic/593-subaru-breakers-for-parts/
I'm not sure how well this stuff works or if you can get to the leak to try it, but has to be worth a look :)
http://www.ebay.co.uk/itm/J-B-WELD-8277-Water-Weld-Epoxy-Putty-2-oz-UNDERWATER-NEW-/330769453745?pt=Motors_Automotive_Tools&hash=item4d036552b1
This is from America. Might be able to get it in the UK some where.
I've also had a look see if I had any workshop manuals only one I have is for the 2001 to 2004 :(