and if so, where are they sold?
Yes, they're legal to own, but you need a special permit to hunt with one. The only place I've seen them locally is at Sports Authority, but it's been awhile since Waikele has had any...I'd just order from Cabelas.
what are the advantages/disadvantages vs compound bows?
It really depends on the use.
Crossbows usually cost more, are bulky and slow to load, but easy to adjust to for a rifle shooter. To get a permit to hunt w/ a crossbow, you'd have to be disabled in a way that prevents you from using a regular bow.
"Regular" bows (compound, long, and recurve) are easily customized, but require a lot of practice. You can bow hunt with a regular permit.
I have used both to hunt white tail, and have no preference for either. They're just not my thing. If you want to learn more and get a "hands on" feel (regular bows), you should check out Island Archer on Koapaka Street.