Ebay,
I used to get it from a place that sold industrial cleaning products but I doubt it’s any cheaper these days. 5l containers last years and seem to be the sweetspot for price Vs postage.
Brake cleaner isn’t one thing. IPA can be a brake cleaner, but not all brake cleaners are IPA. That’s not to say it won’t do the job though, possibly even better than IPA.
Don’t be tempted to use carb cleaner, that’s very different stuff and will strip paint in seconds! I learnt this by using it to clean the outside of my carbs, it took the paint off the engine block, distributed it over everything else, then dried!