Various places really, it's more luck than anything else.
Leaping Lizards in York gets some rarities in from time to time and I got 2 of my Burrowing Frogs from there and then managed to find another 2 on a stocklist for snakes and adders in sheffield!
The Megophrys were just by chance as I go to that pet shop a lot. They are expensive but have been there for over a month and ther's only so much temptation I can take.
If you are a regular somewhere ask who their supplier is and see if you can find a website for them. I've often asked shops to get species in from FaunaImportuk.com!
Also regularly pester Coast to Coast in Darlington to find out when there next shipment is due in - they always get some interesting stuff in!