No the boot can't be replaced. You can try jimmy rigging it back together and getting some fresh grease in there but you will have to replace the axle eventually. I got a used OEM one for $100 shipped a while back from one of the Mazda junkyards in CA, I ended up selling it to Jake.
I'll save you the time in research... Don't waste your time with cheap ones from Rock Auto or whatever, many people have reported that they break. New ones are $300-ish from Mazda Motorsports.
Thanks for the info. I'll probably end up getting one from Mazdaspeed or find a used one. I may jimmy rig it until I can get a new one. Hopefully I can get a new one and install it before the 2nd annual Corey invitational