AlsWings, the problem with filling bottles in scuba shops is not so much technical. Yes, you may have to find a conversion adapter but that's usually not the problem.
The problem is that dive shops and schools, as a sort of gentlemens agreement between them, only sell "dangerous" dive goods, including cylinder refills, to people who have a proper divers qualification.
A regular Open Water diver therefore needs to show his license before he gets his bottles filled with air (dehumidified air, but still air). In order to get a 100% oxygen refill, you have to show a qualification as an enriched air diver, or maybe even a technical diver.
Now IO540 pointed out that if you show up with your aviation license, including the "high altitude" endorsement (or whatever) and aviation bottle, some dive shops will fill your bottle. But most won't.
A regular air refill, by the way, is very inexpensive. Something like 2 or 3 euros for a 12-15 liter bottle, up to 200 bar. I don't know about enriched air/oxygen refills.