You’re the first person to mention neutral collocation facilities as a requirement. The OP only talked about servers generally. Obviously, building your own GPS-based NTP network requires you have visibility to the sky. However, that need not be rooftop access. We routinely locate GPS antennae for time synch at CLEC colos in an available window, behind the glass. Nobody has yet charged us anything more than a one-time cabling fee for that.
But most carrier-neutral colos already have their own GPS-derived, non-Internet time source to which you can subscribe. There is thus no reason to build your own airgapped network, as one is already available from the facility. If you don’t need PTP 50-μs-precise timing for SONET and whatnot, ordinary NTP is very cost effective. For example:
Most colos also offer their own Stratum-1 public NTP servers for free. For example, Hurricane Electric (https://www.he.net/adm/ntp.html). Being you’re already on-net in the colo, that would still give you NTP that doesn’t transit the Internet. Still way better than pool.ntp.org!