Where do I buy the permit? Do they sell them at the park or do I have to buy it ahead of time? Also , I’m planning on arriving Sunday at 10am will there be parking at that time?

You can buy a permit at the sno-park or online ahead of time. It'll save you 10 minutes in line if you buy it on the washington parks site ahead of time. 10am is probably peak time at Hyak and you might not get parking. The lot opens 8am. I've observed that by noon some people are starting to leave, so parking opens up again.

It's a "groomed" park so if you buy online, be sure to get that endorsement.

Buy there