For Napoleonic uniforms, a little would go a surprisingly long way, since a lot of the details of infantry uniforms were common to many armies, including opposing ones, at least during the latter stages of the Wars.   A basic French infantry uniform, if available, could be pretty easily converted to Russian, Prussian, Austrian, Dutch, or Spanish, I should think.   British would do for Portuguese (same design, different colour) and for US (ditto).   The cheeky Yanks invaded Canada in 1812 with "Made in England" labels on their uniforms.  

Cavalry uniforms were a lot more elaborate than infantry, but again, a lot of details were very widespread.   Prussian and British light cavalry clashed at Waterloo, apparently because they saw each other's blue uniforms and assumed they were facing the French (intriguing failure of logic, there).

The actual equipment, to be accurate, would be the hardest part.   One musket does look pretty much like another, but they did vary from army to army, while swords varied from infantry to cavalry and from light to heavy cavalry.

Of course, the wars did go on a grotesquely long time, from Valmy to Waterloo, and every army's uniforms did evolve extensively within that period, as much on account of fashion as from practical considerations.    Headgear, in particular, changed a great deal.

On campaign, soldiers would generally keep their caps wrapped up, to protect them from the elements.   Uniforms would fray and replacing items was dependent on the efficiency of the supply chain.   In one Prussian unit in 1815 there were six different styles of uniform being worn.   There's also the story of Wellington, in the Peninsula, seeing a bunch of shifty-looking brigands shambling along the dusty road:

Duke:  "Who the devil are those fellows?"
Aide:  "That's your Rifle Regiment, Sir."