Sometime in early spring this year I setup up my own Stratum 1 NTP server using a GPS module with PPS output connected to a Raspberry Pi. I took the easy way regarding the hardware, buying a board made specifically for the RPi (not the newer B+ model) featuring a u-blox MAX-7Q module. It fits nicely in most RPi cases, and all you have to do is to connect an external antenna and make a small hole in the case for the antenna cable. The module is available from habsupplies. It does cost about $60 (£36), so it is not cheap, but it works well and is very easy to use.
Most GPS modules I find on eBay only have four connections, VCC, GND, RX and TX. It seems that the ubiquitous quadcopter control systems are not expecting anything more, and the bulk of low price GPS modules seem to be targeted on this part of the market. You can find the at good prices, but to use them for an NTP server you would have to solder in some connection for the PPS output.
The other day I came across something much better, GPS modules from Reyax, Taiwan. They have a web site, but sell their products in their store on eBay. They carry several GPS modules, with integrated antennas, all with PPS output and more, current prices range from $13.50 to $26.00.
Their current top model, the $26 RY825AI, features a u-blox NEO-M8N capable of receiving signals from GPS, Glonass, BeiDou, QZSS ans SBAS (and when available, after a firmware update, Galileo too). It is not a specialized timing module (such as the u-blox LEA-M8F), so the PPS precision is not quite as high as their timing counterparts, but I guess that a PPS precision of 30 ns (RMS), 60 ns (99 %), is good enough for a home network NTP server. (The module used by habsupplies is not a timing module either, and I guess it has a similar PPS precision.) Since you would want to turn off SBAS and stick to one protocol, usually GPS, when using the module for timing purposes, this one might be considered overkill. They do carry cheaper modules that might do the job just as well, but then, why not go for the latest u-blox module, with a TCXO and upgradeable firmware. One cool feature of the RY825AI is that it seems to be able to accept a USB connection, using few external components.
I ordered two of these RY825AI yesterday, and I know that I will have to make some adapter card in order to connect it to eg a Raspberry Pi, and that I will want to use the shortest connections possible. There are ways to tell the u-blox modules about the expected delays in antenna cable (which is very short on these modules with integrated antenna), cables to the destination system, and software delay. Since we are dealing with nanoseconds the speed of an electrical signal in a cable can not be ignored, if you want to preserve the original precision.
I guess that a system with higher clock speed than the RPi would be more suited for this application, to fully take advantage of the potential precision of the time data given. But here too, for a dedicated NTP server on my home network it will probably do just fine. Since even the top Reyax model is less than half the price of the habsupplies counterpart, I do not hesitate to give it a try.
Current link to the Reyax RY825AI on eBay. I am in no way affiliated with them, I just found a good deal and want to share.