Xena46016 is correct, there are no atv transmission in bunker bravo, only atv related items like battery, rubber, carbon composite, etc.
Making deliveries has higher probability for atv transmission then gas station slots In my opinion, and there is no need to do insurance to get it, just to make the right decision when the gas station option comes along in the deliveries and take the safe way after receiving it in order to not lose it afterwards.
There is no way at the moment to buy them in the store, during season it is possible to pay for the premium rewards and get one.