I have never topped up any of my cars fridge systems, though living in the frozen/dark/wet North, A/C is mainly a luxury, unless travelling down South etc. All fridge systems will legally lose, it is claimed, up to 10% of their fridge gas charge by weight every 12 months, though plastic gaskets/seals/pipes, really due to these plastics being slightly porous to these gasses.
A friend used a mobile car AC fixer and was very satisfied with the results, but I can't say how you track them down - yellow pages/yell? These guys only exist on the back of the quality of service that they provide, so if you find one then you know that "he" must be good! Maybe where the mobile fixers work best is if you are not in a rush to get it sorted and they will come and remove the charge, and diagnose where the leak is, maybe giving you enough time to source new or good as new pipes/condenser/evaporator at a price that suits you, then they will come back vac>pressure test>charge if all is good.
Edit:- maybe these mobile fixers suit people with an oldish/older car where going down the dealer fixing route or a fixer with a fixed base would not be practical for costs reasons.