Well, that will not be an easy task. Depending on a large number of factors, the gun is worth up to $10,000-$20,000. If you do have that kind of money, then good luck finding one. There were only about 2900 made, and many of those were used in the civil war. (which means a lot of those could have been blown to pieces, or ended up rusting in the ground)
My advice? Go find a 1873 Colt Single Action Army. They are more common, and cost less.
Want something easier/cheaper to find? Get a M1895 Nagant Revolver. They are historically significant, have a really cool, and unique design, and can easily be found for under $150 in great condition.
IIRC, the Lematt revolver was made in very limited numbers and most surviving examples are probably in museums or private collections. even if one were to come up for sale, the price would not be cheap and would more than likely be sold at an auction with a reserve price.