I had the same problem on my F650GSD in Bulgaria, post Africa, and there with some mechanics we dismantled it and made another bush and oil seal by hand, but this only worked for a few 1000 more kms of tarmac. BMW/Showa do not sell the parts as it is a 'non-servicable item'. Mine was caused by dirt/sand scratching the shaft of the shock and damaging the chrome coating as well as the oil seal and bush. I have since installed a plastic flap to protect dirt from the rear tyre getting in between the swing-arm and lower fuel tank, but I still think the rear shock is a F650s biggest weakness.

If you are going to continue to ride then wind the pre-load adjuster up to maximum to reduce the pogo effect, to make it easier to ride and also so it doesn't heat up as much. I rode like this for about 5,000km more of tarmac and It didn't break, though my rear spring is 2 sizes harder than standard. That said I also once had the rear shock on a NX650 leak all of it's oil (Simpson Desert, Australia) and after 400km of gravel road the shock shaft broke from the lower U mounting point when I hit a wash out (gully/dip) a little quick. I welded it though and continued for 2,000km more but felt like a puppet from Thunderbirds TV series with all the pogo-ing!

*I bought a rear shock from the Motorcycle Breakers/Dismantlers in the UK for £90(~$165), so I could try to find you one again?
*Or a UK made Hagon shock for £255(~$470), as probably anything would be better than the standard shock.
*I think a WP(White Power) would be the best, at £475 (~$870) but yeah, price translates to a lot of Peruvian etc when you're on the road and is hard to swallow.
*Another option I thought of doing while in country with cheap engineering costs is to adapt a second-hand WP shock to fit, from a dismantled KTM LC4 would likely be most possible if the piggy-back reservoir can fit.

Good luck! - Simon