The easiest of course is to take it to your local friendly Scuba shop and have them fill it for you. However, some Scuba shops can only fill to 3000 psi because of the output capacity of their air compressor. Even so, some Scuba Shops will have the ability to “ram” to 4500 psi. If you are unable to find a Scuba shop that can perform the “ram,” you might try going to your local Fire Department.
Fire Departments, even rural ones, have a compressor that has an output capability of 5000 psi that they use to fill carbon-fiber Fire-Fighter tanks. Be sure to check with them first, and if they are able to fill your high pressure air tank with compressed air to 4500 psi, then order your Fire Department to DIN Adapter as shown on the High Pressure Fill Adapters page… don’t forget to take along a box of donuts!
Probably the easiest and most convenient way is to have a high pressure compressor at home. Nowadays it is even possible to own a compressor that you can take with you!