Canon makes an accessory called the "AC Adapter Kit ACK-E6" -- which does exactlyl what you want (slipts into the battery compartment but allows the camera to run on AC power.)
Although the Canon site lists this as a $180 accessory, all major stores seem to sell it for $120 (still not cheap.)
There are LOTS of knock-offs (many of which are $25 or less) but READ REVIEWS because I found the number of negative reviews to be extremely high with many claiming that they either didn't work at all, worked only for a few minutes, or were very flakey (camera would lose power in mid session.)
As such, I'm not sure I'd want to roll the dice on a knock-off.
I have a 60Da (a special version of the 60D produced by Canon specifically for astrophotography) and astrophotography tends to eat through batteries very quickly due to the extremely long exposure times involved. As such, Canon includes the ACK-E6 with the camera (so I own one but I didn't have to buy it seperately). The ACK-E6 does work extremely well.
Tim Campbell
5D III, 5D IV, 60Da
