Hi Their
For Trainz i have often used "Railwaves" whitch i find to have great content.
(I have no connection to to site other than as a customer i should add).
Beware of the above in other replies as there are a number of payware content creators on the forum where the service and quality of their content is questionable to say the least. They do not always state that they are the creators when replying to questions such as yours.
They go under different forum names than their payware sites. Therefore make sure that recommendations come from previous customers with no connection to site
Here is the link to LMS_MAN' Trainz Brittania Payware List in the Payware Anouncements Thread in this forum. You can still purchase his current locos, but at present he will not do any commissions due to other commitments.