Sources for APRS equipment

First on my list is Argent Data Systems.

Argent Data Systems produces the OpenTracker+ and Tracker2 series APRS devices. They also stock a variety of other products for amateur radio operators and electronic hobbyists.

Next would be RPC Electronics

RPC Electronics offers a wider range of products, mostly data and audio cables for APRS and packet radio. They also offer simple serial communications testing and monitoring tools.