In the States ? You can get these from Airspy.Us here.

This is an GPS expansion board designed specifically for the the Raspberry Pi+ Boards (All versions including latest V4).

This board is designed for applications that use a GPS connected via the serial ports to the Raspberry Pi such as timing applications or general applications that require GPS information. To facilitate PPS the time pulse output is connected to a GPIO so you can utilise this board to give NTP PPS discipline. PPS does not work without an antenna. 

This board is equipped with the latest Ublox MAX-M8Q positioning module and high quality filtering capacitors and inductors throughout. The MAX-M8Q features a RTC Crystal, very low power usage and high performance. Although not a true timing module it is more than adequate for a Pi based NTP Server. Its worth noting the module can, with a serial command, be placed in “Stationary” dynamic mode which is the default mode for the much more expensive timing modules.

The board features an SMA connector for an active patch antenna (see related products) power and PPS LED's. On the top is a super capacitor which can be used to retain any custom settings in the event of power loss (If the module is commanded to do so ). The MAX-M8Q is connected via serial to the Pi and the PPS is connected to GPIO18 (pin 12).

A 2x20 way header is supplied and 2 suitable standoffs providing a very robust solution (See additional items if you want additional standoffs). The board follows Raspberry Pi's HAT physical layout with camera and display port notches. It is possible to stack additional HAT's on top of this board with the stacking header.

To use this board to make a Stratum 1 NTP Server please follow these instructions here : https://ava.upuaut.net/?p=951

Datasheet available here

For the communication protocol see the u-blox 8 / u-blox M8Receiver DescriptionIncluding Protocol Specification.

(The command to save settings is CFG-CFG)