does anyone know where to buy 300 grain fp's?

I doubt you will find any that heavy. Is this for wood arrows, carbon, metal? If it is for carbon or aluminum you can get screw-in insert weights and then get 100gr. inserts and with a 125-160gr. tip you will achieve this.

i glued 105 grain,.243 bullets in the end of some venture 300s, 21 grain insert 175 points, over 300 total point weight...

i should have added for axis shafts. i dont tnink a .243 bullet will fit inside. looks like i have a cut and weld project.

caliper the inside,, there are .22,.20, .17 bullets available,,, and easy to add weight for less....