On the Discovery Wiki I confirmed that I could buy mining equipment for my transport but when I flew over to a buy location I couldn't find it in the shop.
(Noticed the wiki says that page is out of date, is there still an area where I can get mining turrets for my Large Transport?)
What is your name?
What is your quest?
What is the airspeed velocity of a unladen swallow?
You want Heavy Mining Arrays, you can find them on almost every Junker and IMG base and a bunch of others. You also want FLStat, to work around the whole "wiki is outdated" problem in the future.
That said, given how the mining mod is set up, I don't think they'll help on a large transport.
Mining arrays (all types) can be bought on most mining faction bases. Besides, FL Companion, FLStat is also a useful tool for finding where to buy stuff.