Where can one purchase RTA cabinets?

RTA means Ready-To-Assemble, so if I'm not mistaken I think you are referring to Ready-To-Assemble cabinets. These cabinets are composed of parts which can be assembled by the end user on site. Some advantages of RTA cabinets would be its affordable price, and that it does not take up a lot of space during shiping and at the construction site.