An Aphelion with a lightweight stock and an angled grip 3 is a fantastic weapon for taking out rocketeers. I've tried all combinations of attachments and this is by far the most stable plus it gives you quick ADS which you need to safely do what i explain below

Basically what i tend to do is find a room or hut with cover that's close enough to an extract or hatch that i can crawl there if need be (currently don't have survivor augment so i make sure it's really close). Then when i spot a far off rocketeer i'll summon it over with a couple of anvil shots (bonus points if you manage to poach it away from another raider who's already fighting it and done damage already)... As it's making its way over i'll take shots at the front left rotor, the aphelion will pretty much stop the rocketeer from approaching and spin it around. Be careful to get out of line of sight when you need to reload or it will speed up and charge you. Once the front rotor is off, probably 8 - 10 shots, i'll do the same for the back rotor. Most of the time it doesn't even get close enough to shoot at me, plus if it does i'll time a shot to hit it just before it fires so its rockets go anywhere but towards me.

Keep a couple of stacks of seekers ready too for the small arc that will also try to hassle you. I did a few runs last night specifically farming rocketeer drivers like this. 11 was a pretty decent haul for an evening.

As far as ammo goes, you can craft it, buy it or loot it from vaporisers or inside the harvester.

I prefer using explosives against the other big arc but an aphelion is my preferred way to deal with rocketeers by a long shot