Take this FWIW because my experience is many years ago. You had to be 'in the business' to get in. I worked as a counterman at a local auto parts store in Las Vegas and nudged one of our HP parts and accessories suppliers for application paperwork. I never had any trouble getting credentials, and after the first year, SEMA sent me a registration packet every year. I think I went to 5 in a row before moving. But I don't know if they've 'tightened up' since then.
So if you know someone who works at a parts store, or warehouse/distributor you might tap them to see if they can get you signed up or at least access to applications.
Comdex (RIP) and CES were actually harder to get into than SEMA back then, but with all the more recent TV coverage, Overhaul shows, etc, who knows what its like now.