Maps are usually available at the conservation office at most of the larger cemeteries in Paris. These are not typically the best guides, particularly for a cemetery as large as Pere-Lachaise.
In fact, there are at least three "non-official" maps available for purchase at vendors just outside the entrances to Pere-Lachaise. The best is probably the Editions Métropolitain map which is usually for sale at the entrance just across from the Pere-Lachaise Metro stop.
ith the exception of the three pdfs of the official guidemaps, which will download the map directly to your computer, the following maps will open in a separate window; simply right-click or control-click on the image: