Damian, as far as I know there are no aftermarket .30 cal ball mounts. There is an Extratech resin kit but it contains an M1917 and M1919 but they are the full guns with tripods and such in PE. One option you might try is to contact Dragon Care and see if they might send you a couple (some people have had great luck with this route).
As for scratchbuilding, it shouldn't be too difficult except for the size of course.

You can use the end of a sprue projection as a starting point.

Here's the Dragon .30 with a cut off nub beside it. The size is near perfect.

Better picture of the cut off piece of sprue.

Once you have the ball part fashioned you can drill a hole down it's centre and insert some stretched sprue. A number of Dragon Sherman kits come with a .30 cal barrel that is only used on certain Sherman types. These parts are usually shaded blue on the parts diagram and are meant for the turret .30. These have the air cooling holes moulded on the barrel. It's on the Sherman 'A' sprue, part #43. The barrel length would be quite fine as I find the ball mount kit part might be a tad long.
HTH,
Jan