You can buy modern marbles in larger amounts which could qualify as cheap.
This is the Marble King site. I assume/hope they are still making marbles in the U.S. Home (marblekingusa.com)
You can find marbles from Vacor de Mexico in the toy aisles of stores. Or here is an online outlet you could try. Vacor de Mexico – Alamo Toy Marble
This is Vacor's site. The company is called Glasfirma now. I had forgotten that. Vacor Marbles - Toys - Glasfirma - Crystal Products Factory
Alibaba is a source for Asian marbles. Marbles-Marbles Manufacturers, Suppliers and Exporters on Alibaba.comMarble