Dollar Tree seems to be everyone’s default for making custom tumblers due to the price point. There are issues though with quality and disappointment with how long they seem to hold up under general use.
Cupture tumblers cost just a little more but are better quality construction, packed so they do not scratch during shipping and they last the test of time! Cupture tumblers come in a variety of styles – clear and candy top and hit a mid point price-wise. A 12 pack of clear costs $32.95 ($2.74/cup) and color ones cost $34.95 ($2.91/cup)
Should you be looking more for a bulk purchase I’d recommend checking out saveacup.com to get a good wholesale price deal.