A couple years ago I went with a smoothbore replica Brown Bess. I've read some say above that people have had trouble with a flintlock when they first start out but luckily I've had no problems. You get a good flint and mount it with a piece of leather and all you need is powder. Most Brown Bess will be in .75 caliber. I shoot both .69 and .75 balls with a lubed patch. Once you get your rifle get onto "Track of the Wolf" web site. They are great for everything you will need to get started.
I bought my Brown Bess from the Middlesex Village Trading Company.
http://www.middlesexvillagetrading.com/ Their reproduction guns start at about $650 and go up from there. I've got hundreds of rounds through mine and it's still running strong.
Good luck with whatever you decide.