I guess pretty much anything that roughly takes the form of a spot 
and emits a sufficient amount of light in the infrared spectrum 
should work. There's also a ready-made (but not cheap by far 
considering the simplicity of the construction and the low cost of 
its elements) sensor bar replacement available from Nyko (and 
available through amazon.com):

http://www.nyko.com/nyko/products/?i=101

This construction is also quite clever (but no longer available as 
far as I can see):

http://gear.ign.com/articles/751/751096p1.html

Have a close look: it seems to take standard DYI-market elements and 
should be really cheap to put together - especially as you'd need 
just half of that bar for Remote Buddy.

Best regards, 
Felix