HNL = Honolulu.
Depends what you need. You can get Aluminum from Royal Metals but it will cost you more to ship stuff to Kauai from here than it will if you buy over the net. Ebay, Amazon, Online Metals and others sell small quantities of whatever you want and many of them will ship via USPS Priority flat rate. The only issue is that the longest piece will be 12" long. If you need longer then Amazon or McMaster Carr are good places that will ship longer items; Amazon Prime will ship long stuff for free so its a very good option.
If I may suggest some materials to start, I would find some 12L14 mild steel, 6061-T6 aluminum and some brass on ebay. All will cut easily and will teach you a lot about machining metal without the grief harder materials bring. As you learn, you can purchase other stuff to try.
I know you're going to be starting with inserted carbide tooling and your lathe will work fine with them. However, I would also encourage you to learn how to grind HSS lathe tools at some point because there will be times when you'll find them to be very useful. My personal lathe is an 11" one with a 2HP motor and I use HSS about 90% of the time, simply because it works better for me.