Broken valve spring... hmmm... How many miles on it, this is a rare failure. I have sprinkled a few answers in your questions below:
My questions are:
* Where can I get valve springs for the car without paying dealer prices or tearing into engines at a junkyard? FCP Groton doesn't list them on their site.
jorrell: Try your local parts store, Napa, Advance, Autozone....
* Should I replace all of the valve springs, or just the one that broke? Should I replace the spring keepers and split collets as well?
jorrell: Replace them all... this is a no brainer.
* If the valve clearances are in spec now, will I need to reshim after replacing the springs?
jorrell: If you just change the springs, reshimming should not be necessary. Depending on mileage, however, seriously consider having the valves ground and the head faced.
* Can I reuse the head bolts, and what torque should I use when tightening them? Also, what torques should I use for the exhaust manifold, intake manifold, and camshaft cap bolts?
jorrell: Get a service manual, preferably Bently. There are way too many torque values to list here.
* Apart from the timing belt, head gasket, intake gasket, exhaust gaskets, and the heater hose that's buried behind the engine (and which I had to cut to remove), should I replace any other parts while the head is off?
jorrell: Gauge the valve guides, they may need replaced. Replace the rubber dampers that go on the very top of the valve stems. Replace the valve seals.
* Am I better off using stock gaskets, or is the Elring stuff that FCP Groton sells ok? This is not a job that I want to do more than once.
jorrell: Elring is fine.
* Should I bother cleaning off what little carbon deposits there are in the motor? What's the best method - wire brush + drill?
jorrell: ARRGH! Have the head tanked and inspected at a machine shop. Don't use anything that is abrasive to clean out the carbon... abrasives have a way of finding their way into the bearings.

You will also want to make sure that the valve didn't hit the piston, if it did, the valve is trashed. Look for a ding in the top of the piston. I highly recommend having the head serviced at a machine shop, this includes refacing, valve guide replacement (if needed), a valve grind, and a real good cleaning.Like you said, you don't want to do this job again!

jorrell