I have a 2015 Lexus GS 350. Is there a warning light to let me know when the key fob battery needs to be replaced

Nope. Like any car, the remote will stop working from long distances first, which is kind of an early indicator. Not really a question about Johnson Lexus though 😁

Nope

I have always replaced the battery myself.

I have a 2006 LS 430 and it does have a Low Battery indicator light on my Dash also my key fob stops working at a long Distance

Where do you go to get replaced?

If your battery is running low you will get a message when you push the start button telling you low battery. It may also tell you to touch key fob to start button.

There is not. You can just replace them proactively. They only cost a couple of dollars.