Inserts are at 3 Rivers, Kustom King, and if you get the GT blems from Big Jim he has heavy inserts too. All sponsors here.
You can also see the GT system on their site.
If you haven't tried it, you might google Jackson Archery FOC calculator and see what you need up front to get your 20%FOC. You can change around values and see what looks good before you buy or cut anything.
You can add weight with common stuff too. Nails, bullets, etc. For example, I drilled and tapped the aft end of some of the 100grn inserts and put an 8/32 brass screw in, 2 inches gave me another 50grns.
Another handy thing is to get one of the inexpensive test kits of target points from 3 Rivers, and you can switch points to try more or less weight. I think the kit goes from 100grn to 250grn points, and has two of each weight so you can mix with different inserts too. That way if you think you are too weak, and are considering cutting off some more shaft, you can try a lighter point to see if that confirms your analysis before you cut. This would also let you try a heavier point weight before you invest in changing the razorcaps.
Hope that is useful info for you.
Walt