LEDs for full daylight

SuperBrightLeds.com have a wide variety, at least. I haven't bought from them much, but they have LEDs in pretty much any size and output you'd want. Make sure you pay attention to viewing angle as well as output power - for a sign you probably want something with a wide viewing angle.

The super bright white LED may be a good bet, only 20 ma or so to light.

I've had good luck with ledshoppe.com. 100 5mm super-bright LEDs them should come in around $5-$8, shipping included. They can take a couple weeks to arrive (they ship from Hong Kong or Taiwan), but all 3 orders I've done with them have arrived reliably.

I'd suggest buying some extras - for only twice as much you can get 200 LEDs and have plenty of extras in case you toast a few along the way.