If you don't need more than "g" speeds, the classic Linksys WRT54GL has external antenna connectors, and it's also compatible with
DD-WRT firmware. Actually, any router that is compatible with DD-WRT
and has external antenna connectors would do.
One of the nice features of DD-WRT is that you can set the the router for the garage to run in "repeater bridge" mode. This will do two things: 1) allow you to plug wired devices into it and have them immediately bridged to your house network, and 2) repeat the wireless signal using a different SSID, providing a strong signal in your garage.
Alternatively, you could use WDS, but I've never had good luck with that. WDS works in a similar way, but also keeps the same SSID on all nodes. However, it's a real bitch to set up, and some wireless adapters can't cope with it.
A nice source for wireless networking equipment is
Fleeman, Anderson, & Bird - they have all sorts of antennas, cables, etc. I ordered cables and connectors from them when I built my
cantenna.