You can look at any Napoleonic range and pick guns from there.
The Mexicans (whilst using British Brown Bess muskets/Baker rifles) used French style cannon. So look at those, I have OG guns, but am looking at Front Rank ones to use with my figures.
OG produce an Alamo range in 28mm so you could use the artillerymen from there, I will be producing them in due course for Boot Hill Miniatures.