I get my 12ga flare rounds from the local Marine supply stores for about $10 per 3 round issue. Go to
www.sportsmanguide.com and look up S/N: DXM7M-74640, New H&K Military Flare Pistol , $49.97 and S/N: DX7M-74641, 12-ga. Flare Conversion Insert, $19.97
For the MKk 79/Mk 81 pen launchers/rounds, check out Ebay, West Marine, Baker Lyman, Houston Marine websites andsee if theydon't carry them. At least one of those will.
My (and my personal favorite) flare pistol is the Navy Mk 5 (10ga) but since those shells haven't been manufactured since 1945, I now own a Navy Mk 5 Mod Bo (which means I made an insert which has an OD 10ga and ID 12ga). It works great! On the initial test firings, I killed and cooked a water moccassin snake with the first round. This is memorable for a former snakebite victim. Payback IS a MEDEVAC sometimes!!!