Depending on how much you want to spend on a rifle check out:
www.trackofthewolf.com
They've got modern mass reproductions to one off hand built replicas and actual vintage firearms to choose from. Spend enough time at the site and you can find some real deals. I highly recommend them, have purchased one firearm from them and perused another that I returned for nothing more than the price of shipping. Prompt fast service, and they have yet to overstate the condition of a firearm IMHO.
www.cabelas.com
Also has a good selection of blackpowder firearms (go to hunting then blackpowder). If you are looking strictly for a 1700s style arm, then you'll want to pick up a flintlock, and most of those in the styles of the 1700s (pennsylvania long rifle etc) use pinned barrels which make cleaning something of a pain in the arse.