Two different issues are at work in this thread. The original poster seemed to be looking for a resonated guitar to use for slide, and other posters point out that any guitar can be used that way. True, but it didn't answer his basic (implied) question.

There are two kinds of resonator guitars, and two kinds of resonators, making for four possible combinations, each of which has its good and bad sides. There is the "biscuit" bridge and the "spider" bridge and the wooden body and the metal body (I've also seen and played a pretty decent plastic body Regal, but haven't seen any of them around since that first one). Biscuit bridges have upright cones (point on top) and are usually thought of as "National" style guitars. Spider bridges have the cone upside down and a metal gizmo (sorry to introduce technical terminiology) on top with a thin bolt running down to the cone. These are the classic Dobro guitars.

Metal body/biscuit bridge guitars are the "classic" slide guitars. These are what you heard Skip James playing.
Some folks get a pretty good sound from the wooden/biscuit models (the National company's less-expensive models).
The wooden/spider model is the bluegrass dobro, and the sound is subtly different from the biscuit models.
The metal/spider combination is a collector's item. Grab it and put it in a museum. Seriously, I haven't seen very many of these, though they have been known to be made and sold.

Ebay prices for these guitars range from $300-$600 for the newer makes (Epiphone, Regal, Johnson, Galveston, Vantage, Dean, Sterling [the name on mine], Liberty, Fender, and others) and for off-brand vintage instruments (I got a B&J Serenader that's a trip -- the "resonator" is about as resonant as an engine block). Everyone has his or her favorite. What I did, and it's a good strategy, is to get one of the Chinese-made metal/biscuit makes (mine's called a Sterling, but it's the same factory as the Johnson, and the Johnson models go for the lower end of that range up there). It was OK, but not great, so I invested in a replacement cone. The best-known of these is the Quarterman cone, but you can buy from National, too, or even from specialty companies like Molinator, an Aussie outfit, which is what I did. I got the cone and bridge for less than $30, shipped from Australia (and it arrived muy pronto, too). Great improvement! Sounds as good as the bottom-end "real" Nationals.

So my recommendation is a metal-bodied biscuit-bridge round-neck guitar (I knew there was another permutation -- square-neck guitars are for ONLY horizontal -- "Hawaiian" -- playing; avoid them if you're playing blues) with a replacement cone. Easy to get, easy to make the change (they're even more mechanical than a banjo -- you don't even have to worry about head tightness), and the results are pretty decent.

Good luck.

Bob Clayton