You can advertise ARIN space. I do it now. But the issue you could run into could be some site may block or restrict what you can do as you'd be seen from an ARIN address.
You can get less than a /21. You need to be the right kind of member. I'm currently setup as a direct access user, but the issue there is that you need to justify the number of IPs you're requesting. Even though no ISPs will accept or re-advertise anything less than a /24, it's not as easy as that. Best bet, if your company will pay for it, is become an LIR. Much easier to request space. But it depends on how large your organization is.
If you need provider independant addressing, that's one thing. Your best bet may be to simply ask the ISP to give you something on the assumption you're in a long term contract with them.
They're somewhere within the last 2 /8s (1.69 as of the 23rd). Once that's reached, it's pretty much game over.