Can you purchase permit while out there are do you have to get it before?

You can buy permits online (https://secure.cloudwisesolutions.com/app/products.html) or at a couple of stores in Arnold (http://ohv.parks.ca.gov/pages/1233/files/SNO-PARK-VendorList.pdf). More info at http://ohv.parks.ca.gov/?page_id=26911

Where in Arnold do you purchase the parking permit?

Or physically you have to buy the permit in Arnold, before you get there, you don't want to go back 30 minutes...

Rei sells the permits.

I know you can get it while you're there, cause we had to they're sold a Bear Valleys general store. There may be a way to buy in advance, but I wouldn't know the site. You can try calling up to Bear Valleys general store they'll know.