You have a few options
If you don't care about the sound and only need the video, you can use an RF to RCA adapter:
http://www.monoprice.com/Product/?c_id=104&cp_id=10430&cs_id=1043008&p_id=4131&seq=1&format=2#description
(also available on ebay for 99 cents including free shipping)
If you have a vcr, connect your roku to the vcr through the rca cables. Connect the vcr out to your tv through a coaxial cable (same type of cable that the cable boxes use)
Or you can buy an rf modulator. There are different kinds so you have to make sure it has a coaxial on one side and the 3 rca on the other. Something like this:
http://sadoun.com/Sat/Products/S/SC172.jpg
Depending where you go, this will probably cost between 5 and 30 bucks. Try to find a mini one, it's good enough and is cheaper.
One warning, some modulators might cause a bit of ghosting so check reviews if possible. Your safest bet.... use a vcr