Jarnagin is a good source, but their "1813" coatee's trim is all wrong and most units do not allow it.
Another good source is
http://www.gggodwin.com/cartgenie/prodList.asp?scat=50Their stuff is great, but pricey (as most good tail coats are), however they do have some overruns from a project that they sell at reasonable prices if they have your size.
All in all, in spite of the bicentennial, there is in reality very little support for 1812 short of little mom & pop cottage industry that is hard to locate and costly.
If you are REALLY interested and dedicated to this you need to contact the 7th Regiment of United States Infantry Living History Association
http://www.cottonbaler.com/They are the best there is. Their CO, Steve Abolt, is the best tailor for 1810-1850's clothing there is, but I warn you because of the amount of work and the quality of materials, he doesn't come cheap...at the same time, its worth every penny he charges. Steve is also usually the American CO at any 1812 event and is usually the garrison commander at Fort McHenry every year.
Chris Fischer
F-Troop