How do you know they are stretched (redacted), fence test?
the actually lines on an RPM are different lengths so you can't just get 'any' new lines unless you measure them first!
you can tune ye olde lines by putting them on different knots on the kite, I remember having a 2,2,3,4 system, except I usually forgot it and always wondered for a few minutes why the kite was pulling to one side

PS.
if you are having trouble getting knots undone you can chew on the knots or hit them softly with a hammer (not on a hard surface)
PPS
basically have to take most of the bar apart but good to get to know your kit
or, check buy and sell, should be able to get something for around the same price as replacement lines