[Elecraft] Suggestions for 10 MHz Signal Source for the K3?

[David and Dianne on Comcast]

Hi Everyone,

I am considering a K3EXREF just for fun, no other 
reason.

Looking for suggestions for a reasonably priced 10 
MHz external signal source.

Can anyone offer some suggestions? GPSDO? OCXO? 
Rubidium Standard?

I have seen some used Trimble Thunderbolt GPSDO's 
on eBay form China. I'm just not sure about the 
wisdom of ordering any of these units.

Suggestions would be appreciated on or off the list.

73 de N1LQ-Dave