Looking for replacement of 4 tube IN-14 Millclock

Well... having not seen your case I'm not sure of some of the parameters the replacement would need to match. Spacing between tubes, for one. There are many makers of Nixie clocks out there these days, for some reason almost exclusively located in Europe. As a matter of fact, this forum is owned by a Swiss maker of Nixie clock kits. Since you said you don't want a kit his products probably won't suit your needs, and I'm not real sure where to direct you. I suppose you could try PV Electronics (England), Nocrotec (Germany), Mr. Nixie (also Germany). I can tell you 4 digit clocks are uncommon and things like tube spacing aren't usually part of published specs.

For front-reading tubes (like your IN-14s) with plug-in pins instead of wires you will need to get away from Russian tubes and look to American (Burroughs, for one) or western European (Valvo, Mullard, etc.) tubes. They are not as common as Russian tubes these days but they do exist. You might find this page (maintained by Dieter of Nocrotec) useful:
www.tube-tester.com/sites/nixie/trade01-nixie-tubes.htm

I find it very surprising that you have had multiple premature tube failures and I'm wondering if you know for sure that the tubes, and not the underlying drive circuitry, are at fault. I've been working with Nixie tubes (hobbyist not pro) for many years and my experience is that they last for tens of thousands of hours. Failures are rare. though more common with Russian tubes that others. Failures I've seen tend to happen right away because of manufacturing defects or rough handling. Tubes have been known to lose their fill gas because of either a defective seal around a pin from the factory or rough handling when trying to straighten a bent pin or force a tube with bent pins into a socket. Something we've been seeing in NOS Russian tubes is internal shorts from the factory, which result in multiple digits lighting at once. Also, a poorly designed clock circuit can cause "cathode poisoning" which looks like the digits are "rotting".

Also, if you happen to want a clock custom designed and built with the exact specs and features you need, I would consider producing it for you. Understand, though, that one-offs are costly because there is no "economy of scale" when making a single unit.

There are a couple of important points in your post:

IN-14 tubes are usually pretty bullet-proof, It's pretty unusual that they should burn out. There are tubes have have short lives, but IN-14 are usually known for exactly the opposite: they tend to last pretty much forever.

Also if you sent the clock back for repair, I would expect you to get it back again: either repaired or replaced. Millclock seem to be positioning themselves as a "premium" manufacturer, and I would keep banging the drum about it until you hear something back. I don't think they are a fly-by-night operation and I would fully expect you to receive the unit back, or a replacement unit, or your money back.

Hang in there. People in this small world of Nixie things can't afford to get a bad reputation.