Does anyone know where I can get LED replacement lights for the kick plate inside the door which illuminate the ST sign? Many thanks

What kind of light do you need?

I suspect that you can visit any specialty shop where you can purchase these bulbs. If you need advice, you can ask those who do the installation.

And in general, there are great sites like Amazon and eBay where people obviously sell all sorts of things, including lamps.
I installed lamps for my door from vont.com, but it's not what you need, but if you like their lamps, you can change your lighting plan. I was very pleased with the lamps I purchased.

I recommend, by the way, taking a closer look at the white diodes with a light temperature of 6000 K.