Does anyone know where to get the huge tips that use in bakeries like BJs and Costco?

I have not seen how big those tips are, but have you tried looking at some of the online cake supply stores?

We don't have those stores here, so my question would be, what are the tips being used for?

I would think the big tips Wilton sells would work nicely. Like M1 4,6 or 8B etc. If you are referring to the wide tip used to ice a cake that is also availble from Wilton - sometimes refered to as the "Cake Icer" tip - I think the # is something like 732.
These are seldom available in places like Michael's but are in cake supply shops, on- line or mail order thru Wilton.

pastrycook suppliers - pastry tips or nozzles ..

OP, did you find what you are looking for?