Why don’t you go and buy the fluid yourself? Then you can bring the fluid to the repair shop of your choice.
I find it hard to believe they won’t sell to a repair shop, it’s how they make money. There has to be more to this story, like the repair shop never paid their bills or were always late in paying when they got parts from the dealer. I have seen that happen to repair shops, they get cut off because they don’t pay on time.