autotrader.com
cars.com
ebay.com

what color do you plan on getting?

what kind of wheels are you getting for it?

Prices on the 2.7 litre models have fallen so low, why bother with the
old 2.5? 2001 models still had the plastic rear window, but they had
the 2.7 litre engine and thicker headlining than the 1997 to 2000
models, without being prohibitively higher priced - I'd reccoemdn going
for a 2001 or newer model. As far as *where* you're going to find one,
try the normal sites such as Autotrader, you may even want to search
ebay. However, you're best bet is going to be to try and find a
"certified" used unit on a dealer's lot. Contact your local Porsche
dealer(s) about their inventory. If you have an automall nearby, you
may want to inquire with the BMW and MB dealers as well, as I tend to
see a lot people moving between the 3 marques and I fairly commonly see
used SLKs, SLs and M3s at the Porsche dealer.