Does anyone know of any online shops that sell just the bare plugs that are used in automotive wiring harnesses? I don't mean just Subaru-specific, but a variety of multi-pin connectors - for example, something like the ECU plugs, or the instrument panel plugs.

http://www.partsexpress.com/webpage.cfm?webpage_id=3&SO=2&&DID=7&CATID=39&ObjectGroup_ID=121

http://www.action-electronics.com/molex.htm

Radio Shack, carries the plastic plugs, male and female, you can get them without wires and put your own in to build just about anything.

Thanks for the help - but all of these places carry Molex type plugs, which are good for generic work.

Specifically, the WRX instrument panel has 3 plugs on the back of it - a 10 pin, a 14 pin, and a 30 pin. These are proprietary, not off the shelf, and I'm wondering if I'll even be able to find them for individual sale.

only if you buy them in quantities of several thousand.

On photos, WRX-style instrument cluster plugs look like standard PCB type, only keyed in a non-standard way. If that's true and you don't care for the keys, you may find at least some of them at Digi-Key (http://www.digi-key.com). You need to know the pitch (probably 10 pins/inch) and dimensions of the plug.
Used them once w. good results. They sell in any quantity, I needed just one plug and a set of pins (sockets).

I had to buy these in quantities of 100 plugs and 6000 terminals each for a project I had in mind. Plus below minimum package quantity fees of 50 per item, plus 100 expedite fee because the total order was less than 3000 USD. Plus FedEx, brokerage, taxes, duties.. Total (for 300 plugs and 6000 terminals) would have come to 2200 bux.

I said "no thank you", and went scavenging at the wreck yard..