There are only a few companies offering low-cost GPS receivers with access to the raw measurements necessary for calculating precision solutions. My work has all been done with U-blox receivers. They are a well established company that also provides easy-to-use free evaluation tools useful for configuration and testing of their receivers and complete documentation. There latest Generation 8 hardware includes both the NEO-M8T intended for precision GPS and the less expensive NEO-M8N intended for consumer applications but also capable of providing the information necessary for precision GPS.
There are other good options out there ... I'm hoping others will contribute more information about them in the community section of this website.