I know papermart.com carries online bakery items in bulk. Probably cheaper to pay for shipping than to spend on gas these days. I don't know of too many places that would carry those kind of items in bulk or if the prices are comparable to business wholesalers.
My go to place is in Pico Rivera. Open to the public and you might end up buying more than just cake boards and boxes. LOL...I always buy more than I should. Its called Bakers Bodega. You can look them up online for basic hours and directions. Last I checked their website isn't all that great. They're also on Facebook and they post info on their classes and free demos too. Their layout is always changing and at first it might not look like much, but just ask if you dont see something you're looking for. It may be temporarily housed in another part of the warehouse while they're restocking other items.
I'd love to know of other places too. I don't need to keep a lot on hand since I'm the family hobby baker. But sometimes my local places dont have what i need when i need it. Hate when that happens. Good Luck!