More than once I've seen people cursing the horrible quality and unreliability of their flintlock rifle only to find out that they're using absolutely crap quality flints or simply don't know how to adjust the flint in the jaws.
No matter what kind of flintlock you get, stay away from sawn flints. Go for hand knapped English flint or chert.
Expensive?
Not hardly.
Sawn flints can run upwards $5 a piece, which is absolute highway robbery.
Hand knapped flints can be had for as little as 75 cents a piece if you buy in bulk, or get a deal at a gun show or through a group buy.
Longevity?
I used to count myself lucky if I could get 25 shots out of a sawn flint before it was trash.
When I finally found some hand knapped English chert flints at a gunshow, I kept track of how long they lasted...
With proper dressing and adjustment, one of those flints took me entirely through a 1-lb can of black powder, and part of another can. At 60-grains of FFG (volumn measured) per shot, it came out to be between 120 and 125 shots.
The other flints didn't last quite as long, but they lasted a LOT longer than the sawn crap.