When looking on ebay go for early '80's GS 450/550 models; they hook right up.
The forks and wheels on REX are from a '82 & '83 GS450L model bike.
I got these forks, and also have another set from an '83 GS550 which have the mounts for the dual disc setup, which will go on REX when I have time to get them cleaned up and check out function.
I paid less than $50 for the complete GS450 fork set, and $35 for the GS550 fork tubes. Both were in very good condition.
Ebay is a great source.