You can buy strings of 50 LED pixels on ebay for making signs with. They are basically LEDs and resistors moulded into plastic plugs that clip into 9mm holes in thin material, and provide a continuous string of illuminated dots when powered with a 5V supply.
The plastic moulding makes them resilient and quite resistant to water ingress. They can be run on lower voltages like 3V if you want a dimmer sign that can be run from batteries.
These strings also make good garden lights due to their genuine effort at being waterproof. You can solder short lengths in place of an existing LED in a solar garden light to power them at night.
They are available in the usual LED colours like red, green, blue, yellow and white. There are RGB and addressable versions in a slightly larger 12mm module too.
To find these lights on ebay do a search for 9mm LED module or LED sign pixels.
In the sets I've bought from ebay sellers the wire has been coloured white for the negative connection and the LED colour for the positive connection. The LEDs are all run in parallel with their own resistor, so you can cut the string to length as desired.