I used to use the harness shown below, sold by Brion Toss Rigging Services. It's basically a very comfortable professional rescue harness, that Brion has slightly modified for rigging use. Highly recommended, but pricey, with the current price at about $380 or so. (Ouch!)
I wrote about using this harness in:
http://www.boatus.com/goodoldboat/upthemast.asp
Basically any rescue harness, as opposed to climbing harness, would work just fine. The difference is in the padding and weight. Climbers put up with thin padding and pain because they don't want the weight, but for working aloft you want to be comfortable to be able to stand long sit times.
A good place to look might be the On Rope catalog, where they have a nice looking PMI harness for $170:
http://www.onrope1.com/store/index.php?p=product&id=111&parent=8
Another source is:
http://www.rocknrescue.com/
The Spinlock harness looks quite good, but from the photos it appears to have much less padding than the rescue models. Believe me, that matters after about 20 minutes! But I strongly agree with your idea to get a good harness, as the feeling of security you get from using one is SO much more than what you experience with a Bosun's chair.