The key is to find a SIP 2.0 compatible IP-Phone. There are many IP-Phones that use a proprietary signaling scheme so that the phones are "tied" to the IP-PBX manufacturer. While some of these may have firmware that can be loaded which then makes the device SIP compatible, there may still be some obstacles to overcome in order to get the full functionality.
Yealink is supported and has a T-19 model that might be somewhere close to your price range. The retail price is around $50 USD, but you might be able to get somewhat cheaper by looking around. Whatever you do, you will be best served by looking for a phone that is on the 3CX supported list as while others can be made to work, selecting a supported phone eases the potential headaches.