If you take any 1805 set (Hät, Italeri etc) you can just make them look a bit more ragged; they'll do fine for infantry. It seems Hat is also releasing a 1805 artillery set, and for cavalry you might try find the old Revell 7YW prussian hussars, or wait for one of the new releases.

I've done what Duco suggests.

Hat 1805 Frech for the infantry, the artillery are Esci Imperial Guard artillery with bicorne swaps. Cavalry is ordinary french dragoons and the SYW prussian hussars for the hussars and chasseurs a cheval. The only piece missing is some suitable generals with wide sashes and big plumes.