I'm sure they could raid some here and there. Considering Tattooine is Outer Rim and considered dangerous, I wouldn't be surprised if the villages or homesteads they raid would contain some.

Of course, this would make the use here a bit weird but maybe they used the blasters for fast moving targets? I image they'd also attack smaller caravans of speeders or something, and for those a slugthrower might not be fast enough.