Where can I find 14"  4 lug rims for a '88 Subaru GL wagon?

Unless you wanna redrill your hubs or do a 5 lug swap, only pugs will do. (Puegeot wheels)

I would think that your best, realistic option (considering your other post looking for tall tires) is to go with 6 lug-bolt rims, such as used on Toyotas, Nissans, Mitsubishis, et al.  Relatively easy, cheap, and allows lots of options for rims and tires.

 

Two ways to go about this is to have your car's hubs redrilled for 6 lug (punch 2 lug-bolts out, and drill 4 new holes), or redrill the 6 bolt rims into a 4-bolt pattern.  Plenty of information on USMB for doing it either way.