I think Dave at TOTW was being overly cautious and didn't want to sell you a part that might not work. You are taking a chance by trying to find a tumbler for such an old lock, but if you really want to get it working again, you need to take that chance.
The tumbler I was referring to from TOTW is part #LOCK-JB-R-TU which fits what they call their Jim Bridger Hawken lock. Their price is $23.50. I would go ahead and order a fly detent with the tumbler to make sure it fits the tumbler properly. That is part #LOCK-JB-R-FL and costs $10.59.
If you order from R.E. Davis, tell them you want a tumbler and fly detent for their Hawken Percussion Lock, item #0211. Ask for one that is all ready hardened and tempered.
Once you get the tumbler, if it doesn't drop right in and fit ok, then you will have to find a gunsmith with experience working on muzzleloaders locks to make it fit.
By "fit ok", I mean the axle on the tumbler that fits in the hole in the lock plate has to be small enough to fit the hole, but not so small that it wobbles or has slop. Also, the thickness of the tumbler needs to be close to the inside height of the bridle. You want to be able to tighten the screws holding the bridle on the lock plate without binding the tumbler. But you don't want too much of a gap between the bridle, tumbler, and lock plate.
I'm a little concerned as to why you have gone through two tumblers on that lock. Other than breaking the half cock notch, a tumbler shouldn't wear out or break unless they weren't hardened and tempered correctly or they were put through some severe stress or abuse. What do think caused the wear and breakage?