I wire up my own harness of lights... the led s are from ebay, or wherever is the cheapest.. they come with free resistors so you can run from a 12v source. but whenwired with a resistor they run ok on 3cells or even 2cells lipos.. ..
its a simple job to do and the weight mostly depends on how light a wire you can find..thats the heavy part...the led and resistor weighs almost nothing..... so usually about 1oz+- a bit for 20-30 lights...
Use your flight battery to power the lights and the plane by simply adding a "Y" lead with a couple of jst plugs... one for the harness and one for the motor and plane..
The lights themselves only draw a fraction of the power that any motor will, I find about 1/3-1/2A so they will cut flight times by just that small amount.. you wont notice it...
Check out the goflykites lighting arrangements they have it nicely sorted
My lights get hot glued to the model... small dabs every couple of inches hold the wiring in place, with a larger dab for the led..
here is my thread for nightkites like these..
http://www.rcgroups.com/forums/showthread.php?t=987191