I don't know any gas station selling Mogas, they sell autofuel. There are some fuels coming closer to Mogas than others, like BP and Total deliver Ethanol-free fuels, but they usually do not open that officially for aviation use.
Most current autofuel (EN) contain ethanol and some additives may be quite bad for the older aviation engines, so one has to do some research, which can be used and which not. More modern aviation engines may be less picky with Ethanol, but the POH will tell you. As most of us will run on STC, the best is to check with your STC issuer which fuels are covered, as the usual Autofuel STC is based on fuel norms no longer existent at tap (or secretly changed by EU buttheads).
I have never come across a limitation of the garage how much fuel to take in jerry cans from tap. As we are a region with quite some farming, they are used to jerry can usage. There are some limitations from carrying dangerous goods laws, you have to keep in mind though, such as: jerry cans are to be carried well vented and outside the passenger cabin, only in cases with certain stickers on it, cans no older then I guess it was 6 years, if carried more than 3x20l with certain dangerous good carriage permit etceteras - in reality nobody really cares, but if you have a road check by law enforcement, be prepared to not speak too loud ;-).