Railway clocks can be bought at auctions and at online stores. If you're thinking of buying a railway clock at an auction then be aware that there are fakes out there. You'll need to do some research on how to spot the fakes. Original railway clocks change hands for a lot of money so there is always a temptation for someone to produce a fake.
There are railway clocks that were made for Midland Railways as well as the Great Northern Railway, the Great Eastern Railway, British Rail, South Eastern Railways and many others.