I cannot seem to find these for less than $4 per tube for a case of them. So, I thought I'd ask for some help.

Unfortunately, fluorescent lamps of any type are rarely sold that cheap these days. Your best bet is to look for them on sale or at thrift stores.

F18T8s were sold as $2 per tube in shops near me, but now the stocks are over. It is best to find them from office spaces that recently changed out their lighting to led. I found a dozen 18w tubes in the trash from an office building, all were slightly used but working.