Can you look through the barrel from the breech? (like AK47, AR15, most bolt-action, but not M1 Garand or M14)
If so, then the best bore sighter is your eye. Look through the bore at a target (I use a roof peak about 100 yards away), then look through the scope. Adjust until the bore and the cross-hairs are on target at the same time. That should get you on paper at 50 yds.
Cost: $0.
(I steady the rifle on my dining room table. Do NOT shoot at your neighbor's house to verify zero)
Regards.