There are the "Aero Trader" pubs sold alongside the "Auto Trader" and
"Boat and RV", eBay, the local paper, and the bulletin board at the
FBO. Also there are local EAA chapters, etc.

Several others as well.

If you have an average spam can in average shape these venues are
probably OK. If you have something of special interest then TAP is
usually the first, best place to advertise. Type or manufacturer clubs
have a newsletter as well, for some aircraft that is the first place to
advertise.