You can rent them at the Fort or Off site (Tee Time Rentals). If you don't have one reserved by now, it may be difficult. They were running low when I reserved mine about a month ago.
You can park by the Bus stop up near the Outpost and by the boat launch at the settlement, however parking by the bus stops near the loops is not really an option, but then, if you have a golf cart, why would you want to leave it at that one.
Golf carts are NOT necessary (unless of course you have certain types of conditions then it could be). The Disney transportation is pretty good, but I always figure on a solid HOUR of travel time from Site to Destination when I use the Disney transportation system. (Bus stop at Loop, to either Outpost or settlement, then transfer to bus or ferry to next stop, etc...). While the GCs are not necessary, they do make some things a bit easier and convenient. Coming back from the Park late and having to wait for the bus (that just left) for 10-15 minutes when you are already tired as compared to jumping in the GC and going right to your site is definitely an advantage. Again, not necessary, but very nice to have if you can afford it.
Disney PO PO is watching.... Always watching...........
We recently rented from Tee Time and I can echo the sentiments of others that they are great to deal with! The 4-seater we rented was fine. The only issue we had was simply getting the battery charger to kick on. The charger had been through some wars. Some wire jiggling and it eventually started. I alerted Tee Time about this and they appreciated the feedback. You do have to pick up the cart off site and return the tow vehicle back to them. They allow you 1 hour to complete the pick up and return (which is plenty of time). Here's a few pics of the cart, tow vehicle, and charger:
