Finding yourself in need of a serpentine belt diagram is a more common problem than you might think. Luckily, there's a lot of help at hand online. For a start, the auto repair forum 2carpros.com has a lengthy selection of such diagrams, including the one below:
Another useful forum for Buick drivers is gminsidenews.com/forums, where users help each other out with repairs and advice. When asked about serpentine belt diagrams, one user recommended gates.com/all-search-tools, so why not give that a try?
If it's a case of a lost manual, there are several 'libraries' for auto manuals online for you to consult. manuals365.com/buick offers the manual for the 1998 Lesabre free to download, as does vadengmpp.com/owners-manual and calameo.com/books.
Finally, if you have the diagram you've been looking for, you might need advice on how to install or replace the belt, so here's a YouTube film showing you how it's done: