Hi Hugh. The only 1/32 set of WWI Belgian decals of which I am aware is a Spada sheet for a Sopwith Camel of Belgian AF Pilot Jan Oleislager, 1918. I think it's OOP, but can sometmes be found on ebay or elsewhere. A difficulty with using national insignia decals generically with WWI aircraft is that the roundels (or crosses) were usually painted on across the full chord of the wings, and this differed from type to type. Makes it hard to use, for example, Sopwith Camel decals on a Nieuport, etc. Some folks just use a circle cutter to make masks and paint the roundels in an appropriate size and nationality.
Good luck with your search! Let us know what you find, as I am sure many (including me) are interested in modeling some Belgian aircraft and would prefer the relative ease of decals over cutting perfect circular masks.

Regards,
Bob