I just checked on that myself...and you can order one right now in all channels for Sprint. Call in to telesales 8007774681 or go to any Sprint Store: www.sprint.com/storelocator -
On your second question...if you want to keep your current #, then we would port it out of your current carrier and in to Sprint. You only have to contact Sprint and sign up and then we handle the rest...we contact your carrier, get your phone number, and then cancel your service with them. If you call telesales and ask about any porting promotions they might have one you can use to get some credit back on your Sprint account to help offset the cancellation fee from your previous carrier.
If you live anywhere near an Apple store, you can preorder online and pick up your phone the very next day starting at 8 am (assuming they have the model you want in stock). The preorder web site goes live at 9 pm every night. Sure beats trying to hunt one down at a Sprint store.