Cesium beam atomic clocks are available on the surplus market, though not nearly as common or as cheap as Rubidium. The clock in the photo is model hp 5071A and I got it on eBay 20 years ago. The tube will last on the order of 7 to 20 years with continuous use, depending on which type of tube is installed.
This was one of the atomic clocks I used for the time dilation / relativity experiment linked at the bottom of the page.
Yes, the original Mt Rainier experiment in 2005 caught the attention of Stephen Hawking and I was asked to repeat the experiment for his PBS/BBC TV series in 2016. It was mid-winter up here in PNW so we used Mt Lemmon in Arizona instead:
Each experiment was a little different; different combination of clocks, different audience, different mountain, different elevations, different latitude, etc.
This was one of the atomic clocks I used for the time dilation / relativity experiment linked at the bottom of the page.