First of all: have you tried cleaning the fork seal? Often times leaks can be fixed quite easily. Remove the dust cap (slide down), then take something stiff and thin (feeler gauge, 35mm film negative, business card corner, ... you get the idea). Slide it in very carefully, then slide along and down (=out) in an ark like fashion. You'll be surprised how much gunk you can pull out that way. Repeat a couple of times until it looks clean. Most leaks can be fixed that way, just be sure not to cut the lip of the fork seal.
If it fixes it, try to get hold of neoprene fork gaiters to prevent it from happening in the future. If you can't get them, you can try packing the space behind the dust cap with waterproof, thick grease to act as a pre-filter. Clean out and replace the grease periodically.
As to where to get fork seals from: in Mongolia, I have no idea (ask in the North Asia forum?), but I know KTM dealers in Germany that will ship out anything you desire to any part of the world. Quite quickly too, if you are prepared to pay the postage. PM me if neccessary.
Finally, if you can't get a replacement, the bike is ridable without any oil in the forks, so no need to abort the trip (as people before have done). I've been told one can subsitute ATF oil if fork oil is not available, but I have never tried this myself. Probably better than an empty fork though. So in the worst case, top it up every now and then?
Good luck, and happy travels.