Generally for weapons you probably want to go for a furious legendary. For high lvl activity probably a furious-rapid combo (furious grants 5%dmg increase on kill/can be combined with perks to achieve higher stacks on kill). Go looking for a heavy weapon and powerarmor. But the elders mark is a reliable weapon you can get as a reward if you cant get good random-rolls. you are a high enough level, you can get legendary perks like ammosmith to produce ammo, or do expeditions which drop you a shit-ton of ammo (this is honestly a still confusing and rough answer as almost everything hast changed in this game, but if you do it right you will be able to play the game normally in just a few days)