@Brownbear
There are some comments about 15mm vs 18mm
Most manufacturers are producing 18mm to fit in with AB, The Warmodelling (now Battlemodels) fit in this category, as do Blue Moon. not sure about Campaign Games, but if you send Dermot an email you should get an answer
If you want 15mm, Essex, probably some others
Battlemodels have nice Nassau, with fuzzy hats for grenadiers, as do Capaign Games. Battlemodels have "generic germans", they are a bit beat up, as though they have been campaigning – those are the ones in Mr Crispy's picture
I would make a list of what battalions you want to do from the lists, and look at the uniforms. You can get a big variety. I did this for Rheinesbund in Spain
Then it depends how much of a stickler you are for uniform accuracy. If you use 1815 Nassau from AB for instance, they would be at home in many other campaigns. You could paint some 1815 Dutch – there is bound to be a very close uniform somewhere
Not sure whether any Rheinesbund in Germany were still wearing bicorne at that point – you could use early Prussians for those
Hope this gives you some ideas
John