Thanks everyone for responding. I've spent today making more phone calls and not really getting anywhere. I've called the bigger stores like Home Depot, as well as the little stores that specialize in lamps and bulbs. No luck. I also called the number that is listed on the instruction paper that came with the lamp, not really expecting anyone to answer. To my surprise, it's still in service, though I'm not real clear whether the business it's associated with is the one that originally offered this lamp (they're called Evolution Lighting, I believe). Anyway, he checked and, no, they do not have the needed bulb, but he recommended three websites that I could check with which I hadn't gotten to yet.
One site--fullspectrumsolutions.com--had it listed under "replacement bulbs," but at $29.50 for just one bulb (plus shipping). Too much! For anyone else interested, the listing specifically states that it's a replacement bulb for the QVC lamp. It says it's 55 watts, but is 6500K instead of 6400K. One thing, however, I'd want to be sure of before ordering this (if I were so inclined) is whether it's definitely made to use with a dimmer switch. Just a warning to any of you who might consider buying this at such an outrageous price.