Where can I find:"Marui G3 selector"
 

I lost my Marui G3 selector. Who can help me??? THANK YOU

selector plate 1
selector plate 2

and if you're looking for the lever:
here

Thank you. But do you have other brand suggest for me??? I think original lever is the best.....where can find it ???

here
Currently out of stock though. Register an account and you can have them notify you when it's in stock again.

Next time just go to any big airsoft retailer like redwolf and search for what you're looking for. You'll more than likely find it. If that fails, there's always google ;)

For reference, here's a review of the TSG G3 selector switch: click

After some brief research, I found that the most reviews say the original Marui lever switch is flimsy and prone to fall off/break. The TSG version of it seems to be much better as it's got more metal and is less prone to breaking.