Two Panasonic NCR18650B unprotected batteries with case can be purchased for just under $13.00 on Ebay. You mentioned you had a meter and could check the voltage of the battery. When you get these they should be at a charge of around 3.62v (ie: that's the voltage when they ship new). Run your light at this level and see what it does. If you can get all your modes, then run it on high for 5 to 10 minutes to drop the voltage. Check with your meter to get a feel for how fast its dropping. Get the battery down to 3.00v or even 2.80v and see what your light will do. If its obvious to you that you're losing the higher power modes, then you might be fine with the less expensive non-protected Panasonics because you'll know far in advance that you shouldn't be running that light anymore without recharging that battery.
If wanting a protected, (Eagletac and Nitecore lights require them because of their requirement of button top), then the ones that are probably the most popular use this very cell (Panasonic NCR18650B) and add their protection circuit to it. There are several good brands that use the NCR18650B. Olight, EagleTac, Keeppower among a few. Just shop for who has the best price. Hard to beat the $10.99 Keeppower at Illumn. In the past, the time that I have purchased batteries has been when I was piggybacked onto another order thus the shipping was negligible.