There is a company called Jackson Labs that makes suitable GPS modules, I have heard of customers using those products. We have used a ublox EVK-F9T eval kit and Connor Winfield module in the past. The CW module suffered from the 19.6 year rollover issue, so if you bought one used it might suffer from this.
If you aren’t aware, our SP145 and SM200/435 receivers have a built in GPS for timestamping and disciplining. We have certainly heard about the lack of suitable external GPS modules and the SP145 is becoming popular for this reason.