The simplest and most basic (and the earliest) is the ST-6. The Dovetron MPC-1000 offers the most flexibility (it is what I use most of the time with my M19 and M28ASR). All these TU's have built-in loop supplies. Any one of them will get you going. There is a lot of information online (manuals, etc.) about these TU's. Google is your friend. There haven't been many TU's for sale on eBay lately, but they show up every once in awhile. Much of the time they are in working condition, but might require some repairs.  You can also put up a query on Greenkeys. Maybe someone has one for sale.

73,

Bill

KE3BK