After purchasing two UV meters and a Mercola stand-up tanning bed, I'm can now shed some light on my own question.
First up: natural sunlight. Here in the vicinity of Washington DC, my measurements consistently show that UVB is about 7% of the UV spectrum (the rest is UVA). It varies a bit depending on time of day and cloud cover, but it's always close to 7%.
So I started looking at stand-up tanning units. The beauty of these things is that you can stand as far away from them as you want, so that you're not getting blasted with UV radiation that is stronger than what you get from the sun. The bulbs in Mercola's units are from ESB (www.esbtans.com). Before purchasing the unit I asked both ESB and Mercola's people what the UVB/UVA ratio of these bulbs is. ESB said 5%, Mercola said 10%. (Good information is hard to come by. :-) I figured I'd be good either way given my readings of 7% from the sun itself.
The unit arrived yesterday and I couldn't wait to set it up. The UVB percentage is 10%. The UV radiation as a whole (when measuring right up against the unit) is about 3-4 times stronger than the summer sun at solar noon here in DC. I have no desire to subject myself to this level of UV simply because it isn't natural. But I can get the UV levels that I desire by standing about 3 feet away: this gives me a level of UV roughly matching what I was measuring from the sun back in mid July.
So, I'm pretty pleased with this whole situation given that winter is right around the corner.