Yes, a "google search" for SL82218 returns quite a few loose handsets being sold on eBay. But then you'll have to deregister
the new handset from its old base and re-register it to your base...

1. When the new handset says "Connecting..." and goes nowhere
else, enter *331734 [menu] and it will soon beep and display
"Not registered"

2. To register the new handset to your base: on the base press
the "Handset Locator" button and hold it until the "in use" light
comes on ( ~ 4 seconds ), then release the button and set the
new handset into the base phone cradle. After ~ 30 seconds
it will beep and display its "Handset #" (next in sequence).

3: Note: the base has a limit to how many handsets it can handle,
if your system was fully populated you will have to clear all the
handsets from the base and re-register them ( it is the only way
to 'rescue' the place occupied by the lost handset ). The manual
describes how to "Register a HS" in Appendix C, pages 60-61
and it is available from: www.manualslib.com

Good luck !