If you get a 100K pot, you are not limited to digikey or future.
The supplier that I like for things like this is mouser electronics (
www.mouser.com). They have good selection, quick shipment, and no minimum order. Although, a quick look through their catalog did not disclose exactly the option you wanted.
Another approach might be to get a Grayhill gray-scale mechanical encoder from Digi-key. This can be connected to four digital inputs (16 bits per revolution resolution or one nibble), and gives you unlimited rotation. I don't know how fine a positioning you need to do. Of course, you could gear this and accumulate your value in software to improve your resolution.
I would definitely recommend against using a 300 degree (maximum) pot for 300 degree rotation. Your accuracy is very poor at the ends of the travel and if you do not position the pot exactly, you will break it quickly.
We normally use the 270 degree rotation pots for about 200 degrees of rotation.