My '87'* a/c compressor went out a looong time ago. (circa 5 years). Our mechanic (i use that term loosely) removed it and put on a shorter belt to exclude the a/c compressor.
All you gotta do is get a belt made for your engine but w/out AC.
See if you can find any such belts via a car parts web site like advanceautoparts.com or autozone.com etc (most let you find parts through a system identical to that which the parts guys use at their stores, trust me i work for one such store. And that should give you a part number, etc to go to the store and get it or even to order it online.
Oddly enough, for my 87 bonneville i can't find a non-a/c serpentine belt,, this is probably because all 87 bonnies came with a/c and so the parts catalogue won't list a non-a/c belt. However most other GM 3.8L engines from your same year car without a/c belt should fit fine, its just up to you to find it.
Once you get a blet the riight size for what you need the rest is easy, just route it as per that drawing under your hood and you'll be up and running.
87 Pon Bonne.