For now I'll just buy spokes individually from the LBS. They are $1 each, which isn't the worst thing in the world. They do sell them online for about 50 cents each, but with unreasonable shipping charges. What can I say, I'm a cheapskate. No one here seems to have mentioned any bike component sites that charge reasonable shipping charges for something as light and small as a dozen spokes, so I'll just stick with the LBS for now.
I have no plans to buy a new wheel since the one I use isn't that old at all and I have absolutely no desire to spend $200+ on a decent new wheel. Yes, I've broken so many spokes at this point - I've replaced 7 of them on the rear wheel so far, all on the derailleur side. It's a good idea at this point to get the wheel straightened at the LBS, perhaps after replacing all the spokes on the derailleur side of the rear wheel. I have checked the spoke tension myself by plucking them like a harp, and they don't sound under-tensioned. I used this method to also even out the tension of the spokes. I had replaced 5 spokes on the rear wheel, and for about 7 weeks I had no broken spokes and used my bike frequently, sometimes with heavy loads. That changed this week when I broke 2 more.
I'm pretty sure the rim isn't bent. I do things like carry 40 pounds of groceries on my bike and ride on metro Boston city streets. But more importantly, I dislike leaving my bike for repairs and having it in the shop for 4 or more days. I don't own a car and my bike really is something I depend on every day, so I prefer to do all repair myself. Right now I don't have a second back-up bike. Although it's gotten to the point that I'll have to leave it for repair anyway and swallow my pride and take the bus.
I have noticed that my old spokes were some off-brand (they have an "R" logo on the nub-end of the spoke) and my new spokes are both Wheelsmith and DT Swiss brand. I don't know if this will really make a difference. In the future, can I mix butted spokes with the straight spokes I have on now? The butted spokes supposedly resist breaking better than the straight ones, from what I've read.