If you are VERY tech savvy; consider re-flashing a Linksys WRT54GS
(or WRT54G-TM) with DD-WRT and [re]configuring it to act as a repeater.
If you are not VERY tech savvy; buy the kit from JefaTech:
http://www.jefatech.com/c=fq1xxzn24y...category/a050/
-doug