Hey there fella...I actually am in the process of doing exactly what you want to do. The best I could find in a reasonable price range was a Sharp LQ104V1DG11. It's 350 nits and requires 2 backlight inverters for full brightness. I'm hoping it will be much better in sunlight than my current 200 nit screen. I'll let you know how it goes.
I found that screen when browsing
this site
The salesperson there quoted me a ridiculous price of like 300 or 500 bucks just for the SCREEN! I think e-bay will definitely be your friend here

I suggest you go on e-bay, look what screens are being sold, then try searching for the datasheet for each one and check the brightness in nits (cd/m^2).