No idea where to source it at scale but it looks to me like a Barrett M95 AMR with a heavier carriage in place of the stock. If you found one at 28mm scale it might suit the “cannon” sized unit you see there.
If you can find one on ebay Roco minitanks 1/87 German Flak Sdkfz 7 and SWS halftrack had a 3.7cm gun mount that would do you. It also came with a Flakvierling 2cm Flak as well. I used the spare guns to make 1/76 2cm guns for my Panzer Grenadiers. You might be lucky and some kind soul may have a spare in their bits box. One of the 2cm guns might do as well. I am sure that I used all mine but will have a look a the weekend.