If you don't know then chances are you don't need them, no offence meant by this, just my own opinion
FYI - the only time they are used in road races like the TDF are during time trials, and the bikes they use on these stages are very specific and designed for one purpose.
That said if you do want some, (although they make the front end steering feel heavy), then have a look
HERE
I wouldn't buy new though, lots of people get them but find they don't like them and sell them on quite quickly, so there's usually bargains to be had.
Also, they are usually not permitted in any organised rides you may wish to do, although how strictly this is enforced I couldn't say.
[EDIT]
@rebelpeter - FYI
@Cyclist33 has a pair of TT bars for sale
HERE.