EDIT: If you don't feel like the rigamarol, you can use this quick-n-easy method, but lapsey's script works much better. See two posts down from this if you can't find the uca.xml file mentioned in the HowTo.
I installed the gnome-utils packages with synaptic (about 8MB for me).
Next I opened Thunar (Applications>>FileManager), then opened the command editor (Edit>>Configure Custom Actions) and added a new custom action.
I put the follwing info in the Basic Tab:
Then in the appearance Tab I left "File Pattern:" as "*" and left only the "Directories" box checked.
Name: Search for Files
Description: Start file search here
Command: gnome-search-tool --path %f
Close Thunar then start it back up. Right click on a folder to search.