I wouldn't recommend any batteries that are not Dewalt-branded. There are no reputable generic batteries. They may power your impact gun, but lack the heat and overload protection of the original batteries. It is very common for them to be mis-labelled as having higher capacity than they actually do.
You can fit any Dewalt 20v or 60v battery pack in your impact gun.
60v packs (aka "Flexvolt") actually run at 20v in 20v tools and then 60v in the 60v tools. It's unlikely you will want to buy any of the 60v packs for an impact gun however, because they are physically large. A 20v pack doesn't run in a 60v tool though.
Any Dewalt 20v pack will fit, the difference is just how much capacity each pack holds.
- 1.3ah and 1.5ah battery packs aren't usually sold except as part of "kits".
- The smallest regular retail pack is 2.0ah
- there are "regular" 3.0 and 4.0 batteries - and also 3.0 and 4.0 "compact" batteries. A 3.0 pack has the same capacity either way, but the compact version is more... compact.
- Once you get to the 6.0ah sized 20v pack, that's getting large for use in an impact gun. The 6.0ah 20v pack is the same size and capacity (pretty much) as the smallest 60v pack.
If you really want, you can connect even the new 15ah 60v pack to your impact gun - but that would be a workout.
I should add: there aren't any other 20v packs from other tool brands that are compatible with Dewalt. E.g. Craftsman, Porter Cable and Black & Decker 20v packs do not fit in a Dewalt tool.