Thanks TeriM, I'll look into that. My first thought was to pick a supplement matching as closely as possible the ingredients of the food my Vet promotes.
I couldn't get a dispenser for the bottle, they weren't the right size.
Something else I tried to get was powdered G/C/M and they didn't have that either. I can get it at a local tack shop, but it is not flavoured, and I have to buy a quite large container as it's meant for horses. I'll end up throwing most of it out, making it very expensive.