[Arran]

 With most output tubes, 42, 6F6, 6V6, 6L6, 6BQ5, you can use a transformer suitable for one of the other types and it will work fine. For the 6K6 you could use one suitable for use with a 41 or a 6AQ5, as there are basically variations on the same tube, not sure about Loctals. Edcore has a good selection of replacements, some of the universal ones will work too but cost more more for including the extra taps to very the primary to secondary ratio.
Regards
Arran