There's a couple of shops that could deliver on order, but I don't think anyone is likely to have 8x10 on stock - at least not consistently. Your best bet would be to mail order from fotoimpex.de or macodirect.de and have it delivered to your hotel/accomodation. If you get the timing right, it may just work out, but it'll be tricky.
You could also give the guys at Grobet a call (grobet.be). They don't list any 8x10 on their website, but if anyone is likely to have any lying around in Belgium, it's them.

Finally, you may want to shoot Matheiu Bauwens a message; he's from Belgium and also shoots 8x10 IIRC. He's probably the most likely user on here to be able to help you out with some pointers.