1.
Pentax ring flash at ~$500
2.
Sigma Electronic Flash Macro EM-140 DG at ~$400
3. Alternate models that fake it by channeling light from your popup flash or a standard hotshoe-mounted strobe at ~$65-150.
Which one depends on your macro shooting skills, your standards, and your budget. If you are just learning how to photograph closeups, and are uncertain whether you'll get into it, then go cheap and grow with your capabilities.
This would also apply if you are not all that proficient with flash shooting in general.
I'm comfy with both closeup shooting and strobes; I chose the Sigma. I got it new from a vendor on the big auction site for $245, but I think he's out of stock. Anyway, it's a great flash, lots of PTTL and manual controls, well-built, easy to pop on and off lenses, and just a lot of fun. I really like the flat, full light. It's especially fun if you relish in post processing.
I've used it for beyond closeups with the DA 21mm Ltd.
I just don't know if the fake-out ring flash gizmos can blast out that much light, but if they can, then they may be a low-risk try.
HTH
M