For those of you who game in 15mm some of the companies I would recogmend for both service and minature's
Peter Pig, for both figures and buildings. Peter pig minature are true 15mm and may not match in with other producers, but that does not realy matter as they have all the figures you will need for the first two books (The Old west and Pony Wars.
If you want to do the Alamo I would surgest Minifigs (c/o Cavalier books) they have all the various figures you would need, but the figures are about 17-18mm tall.
A great source of civillian figure for the Old West is Blue Moon minatures in the USA, very nice figures.
Finally for the French and Indian wars there is Essswex 15mm who have a very nice range of figures for this period but expencive, so if you want something cheeper that still is of a high standard then I would recogment Iregular minatures.