This happened to me yesterday. I went to Verizon, but they didn't have an external battery dock/charger. Also, I don't know anyone with a compatible phone, so I was literally unable to power up my phone enough to try flashing again. I found a post on another site, xda-developers, and was able to rig the thing myself.
This is what you have to make. Believe me, it's not very difficult. All you need is an old USB cable (Standard, Mini, Micro, doesn't matter), a pair of wire cutters, and some tape.
1. Clip the connector off of the device end.
2. Strip about 2 inches of the insulation off of the cable.
3. Once you remove the shielding by just unwrapping it, you expose 4 wires.
4. Take Green and White and fold them down and tape them to the cable to get them out of the way. This leaves you with red and black.
5. Red is + and Black is - or ground. Strip some of the insulation away (about 1/3 - 1/2 inch or so) and expose the conductor on the red and black wires.
This is what you should have...
6. Place the wires on their respective + and - terminals on the battery, careful not to touch them to the two other terminals, and then put the battery back in the phone. YES, put the battery back in the phone. It might be a tight fit, but if you exposed enough of the conductor, you can do it. Trust me.
7. Plug the male end of the UBS cable into your wall charger with USB adapter. You can try plugging it into a spare USB port on your computer, but I wouldn't recommend it. Apparently cutting the cable end messes with the flow of current. My MacBook gave me a message along the lines of "A USB device is drawing too much current, so all USB devices have been disabled. Unplug the offending USB device and USB ports will be re-enabled." So yeah, plug it into your wall charger.
8. Take a separate USB cable and connect your phone to your computer through the standard charging/data port on the side of the phone. You should now be able to boot up the phone in AP Fastboot mode, and get a "Battery OK" status. Basically, you're making jumper cables and giving your phone some juice to get going.
I wouldn't recommend doing this for an extended period of time. I monitored the situation during the entire flash, and did not notice anything getting hot (or even very warm). However, please note that doing anything like this is bound to be a fire hazard, or at least pose a risk to your phone and especially your battery. My experience was great and the cable was exactly what I needed to get my phone going. However, you should do this at your own risk.
Advice to take away: Never, ever attempt a flash unless your battery is at 100%. If something goes wrong, and inevitably it will, you'll be glad to have extra juice to correct the problem. Hope this helps some of you who were in the same situation I was in last night.