If you are looking to buy one any machinery supplier can obtain one and would be very happy to do same.
If you are near a port with a ship reapair facility most have lathes in your range.
When contacting anyone make sure you have all the necessary specifications for your requirements. There are considerable number of paraments that will determine the cost if new and who can do a job for you.
Come back with a few more requirements and someone will point you in the right direction.
Try Kvaerner at Sheffield, when I used to go in there between 10 and 15 years ago they had two large lathes for turning Rolling Mill shafts. The largest, if I remember correctly, was about 40 feet between centres.
In those days the firm was called Davy McKee and it had very big machine tools and as far as I know it still has. They used to welcome outside work although their main product was design and manufacture of steel rolling mills.
Kvaerner
Prince of Wales Road
Sheffield
S9 4EX
United Kingdom
Telephone: 0114 244 9971
Fax: 0114 291 4008