I have been eyeing the SU-39 and the Type 90B Fuji, I would like to know if or when they will get a sale :)
It will be announced at Official News, Development Blogs and Updates under the tags shop, these sales usually happens right after PC sales but may be cases that it can happen before. Individual sales may happen too so you should take a look at national events that may promote said vehicles.
Next major sale is Christmas IIRC