There’s an agent I’m interested in. but it is listed as referral only, how do I submit without knowing someone to refer me?

Having a referral is always best, but I would suggest submitting your photo and resume anyway. The worst thing that can happen is that they press delete if you send your info by email and toss it if you send it by snail mail. Who knows, you may get a call if you just happen to be what they are looking for at that particular time.