Cheapest is to only use one per bar. After all, the primary purpose of adjustable end links is to get rid of preload in the bar, and you only need to adjust one to do that.
But the end links on an NA Miata also have a rubber bushing, and this can make it very difficult to reach certain holes in adjustable bars. So there's a potential side benefit if you use an end link with a ball joint or a rod end.
The Gearhead Garage ones are no longer available.
The ones sold by Go Miata are actually from Racing Beat. Note the urethane bushings - doesn't help with the range of motion, but it means they'll never wear out.
There's an excellent chance that the "Mazda competition" ones sold by Goodwin are actually the AWR units.
Stay away from the Moss ones, resold by a number of other people. They're very cheap rod ends and will wear out in months.
Flyin' Miata sells Whiteline end links. They're not cheap.