How can I charge just one Lithium Polymer cell?
Search on Amazon.com for "lipo charger". There are several options that will charge a single cell (1-4 cells, 1-6, etc.). Click here for one example.

Just be aware that many R/C-oriented chargers do not include a power supply, as they're intended to be hooked up to lead-acid batteries when out and about. (The linked item does not need an external supply.)