Powder
I use a powder coater local to me and he used to doing cycel and motorbike frames, so it's more like paint than powder ,but he is pricey, compared to others, To be honest, I would recommend you do it yourself.
It's not that hard!
First strip and buff the frame with phosphor bronze brushes and Scotchbrite, then degrease with Meths, thoroughly.
Buy a Badger pro Airbrush from Halfords, these are about £50 and as they apply an extremely thin coat, it's easy to build multiple layers up, without any runs, use a good rust barrier undercoat, generally Zinc based, build up in thin layers. (The colur will affect the top coat colour, so choose accordingly.
Then apply a top coat of choice, there's loads of quality paint available now, for not a lot of money, most gets a good finish without lacquer.
Again build up in thin layers, it's all best applied in a warm atmosphere!
Maybe best practice first on something like old tins etc.
it will be much more rewarding than paying someone else and cost less too,
It's important that you give it a really good degreasing first!