This blog has a few lasgun to autogun conversions that are fairly simple. Personally I use Maxi-minis Ak-guns with my Imperial Guard, though if you are buying from Maxi-mini I'd note that their stuff from Kromlech is not scaled to humans, but for orks, and that their Aks are a little chunky.
The Assault Group also produce a large number of real world weapons that are compatible with Warhammer 40,000 humans models, though as typical with most other companies selling weapons they are lacking a bit in the details.
So yeah, Maxi-mini are probably your best bet if your looking for detail (and that's not saying much), and their probably one of the cheaper options too. =P