Proxys just make your accounts connect to jagex with a different IP, so in theory Jagex wouldnt be able to ban your other accounts if only one gets detected. But no one here workls at Jagex so thats all just assumptions, so its up to you to test out how many accounts you can run on how many IPs.
Maybe it isnt even worth it with whatever banrates your methods have.
Idk where the best place to buy from would be just google for cheap private proxys until you are satisfied
To set up proxies literally just launch osbot and you see it: