I don't think I've seen a 6 pdr with a specifically sikh crew, you're most likely going to have to head swap.
Best is subjective and I don't own one so can't say, but companies that I can think of off the top of my head that sell them are Warlord, Perry miniatures, Artizan and Crusader. (both available through Northstar.) As for US distributors, Warlord have their own site for over there and the others I think might be available through brigade games, though I could be confusing this with something else.
Hope these ramblings help.