You can get them online (
shopdisney.com, then search Magic Bands). The Shop Parks smartphone app also has them. Not sure if Disney Stores have them.
As far as if they're worth it ($12.99 minimum each), don't feel required to get them. If you don't mind taking your admission ticket out of your wallet/lanyard/etc, there's not an overwhelming reason to get them. There are a few rides that use proximity sensors to get pictures into your PhotoPass account (Mine Train comes to mind) and a little magic that happens at a couple restaurants (Be Our Guest, maybe Satu'li Canteen) so they can find your table easily, but in most cases, especially for offsite guests, tapping your card is equivalent to tapping a MagicBand. They're a nice to have, but certainly not a requirement.
Remember, the linking you mentioned that takes place is to your My Disney Experience account, not a specific ticket or MagicBand. (And another poster correctly mentioned that since you're offsite, you won't be linking any credit card). Once fastpasses, dining, etc. are linked to your
MDE account, you can use a magicband or ticket interchangeably.