Do not do this. You will just annoy them.(Emailing professors at your own institution probably is okay.)
Pre-doc opportunities are posted at the NBER. Instructions on how to apply can then be found for each opening..
You can also Google for predoc programs that may not be on NBER, but along the lines of what startz says, even if you get replies from professors they will need to see all of your credentials and compare them with the rest of the applicant pool, so you won't be hired on the spot at the very least, and depending on how canned your email is you might end up diminishing your chances by appearing rogue without some kind of alluring credential to provide reason for the need to reply.