Columbus Max forks are my favourite forks, I have them on 4 bikes, sorry they are all on bikes that are in use so don't have any for sale. I too am in the UK and they come up pretty infrequently/never here, and tend to fetch a premium. Coupled with the fact you need one with quite a long steerer, be prepared for the long haul, but they do come up, though I'm not sure you'll ever find a 'cheap' one.

As you said keep an eye on overseas sales, though post-Brexit this has already got more expensive. For the two bikes I bought the forks for separately, both for 57cm frames or larger, I had to wait over 2 years of searching eBay before I found anything suitable. I found this one on Italian eBay, so always worth keeping an eye on there as Columbus is Italian obviously.

And ever since I bought my ti Merckx I've been looking for a Max fork for it to replace the old carbon one it came with, and that frame is larger that 58cm so that was a long wait too. Only in the last month or so did I eventually find this beautiful fully chromed one, and that was from Western Siberia of all places.

So don't lose hope, they can be found, and the longer the wait the longer you have to save up, good luck!