I found the Trakz TX0004 LVT-A1 Detail Set for $14.95 (normally $32.95) at my local hobby shop. It includes a Jordi Rubio aluminum 37mm barrel, resin .30 cal machine guns, resin upper hull (top portion of upper hull with the hatches), and a PE fret with .30 cal gun shields and mounts, hatches, .30 cal ammo boxes, grills, periscopes, and those little grill like things at the back of each track.
I do know that Verlinden sells resin .30 cal machine guns (#371) and Jordi Rubio sells the 37mm barrel (TG-48) separately as well. This would run about $10 plus or minus depending on where you find the items.
I got lucky with my purchase. If you can find that set, I recommend getting it though. But $33 is a lot of money to spend.