Check Hyperlink Technologies for inexpensive directional and omni-directional antennae. You can order them with the right connector for your WAP (ie...Linksys uses RP-TNC). You can probably get two old Linksys WAP11's cheap and put them in bridge mode.

I would do as Tony suggested and just put them in a window sill with line of sight. That way, you don't have to worry about grounding the antennae. I would, however, invest in the antennae, seeing as you can get both for under $100 and it helps guarantee you get a strong signal and full bandwidth.