New ones from Siler, Davis and L&R are up into the $130/150 range now unfortunetly. Buying parts to 'build' a rifle is going up.
Quite often a 'new but never used' lock can be bought second hand for a little less, and maybe including the shipping.
A lightly used lock or one that has been fitted (drilled for the side lock screws but the rifle never completed) goes up for sale for around $100+.
I often see them on the ALR forum (American Longrifles Forum) in the classifieds section. They go fast though!
American Longrifles - Home - a site dedicated to the study and building of the American longrifle
click on ALR Forum ,,,then 'To Classifieds Forum',,,then to 'Items for Sale /Wanted'.
Good bunch over there.
I'd stick with a modern recent made repro from one of the above mfg'rs.
There are plenty of Spanish, Belgian and Japanese made repro flint locks out there from the 60's and 70's and even newer (Maslin (Spanish), Dixie Gun Works locks from the 50's & go's were Belgian for the most part). Many were serviceable locks but not top materials built for long service. I used some of the Dixie locks to build rifles in the early 70's.
But they are getting near 50 y/o now and worn. They can be fixed and some parts are available.
The easier soln for those is generally to buy one of the several 'replacement' locks that L&R or Davis makes that simply drop-in and match the inlet for these popular older locks. But you are back up to the above new lock prices again..
Any original hand forged F/L in decent condition is a collectible in it's own right and should be preserved as it is IMO.
Ebay is another source as always of M/Ldr parts and locks as snubbyfan points out. Sometimes the bidding gets crazy, but with some searching and luck you can find some decent deals especially if there is a Buy-It-Now at a good price and you happen to be the first interested person to see it.
There is lots of info on that ALR forum in the 'Gun Building' section. Personal opinions of certain mfg'r locks and styles are discussed quite often and the Forum has the same demeanor as this one does.
Dennis Glazener runs the ship over there and does a nice job of it,,and builds some nice rifles too!