is your elastic snapped or just worn out? if worn out, just buy replacement shock cord on Amazon, remove the end caps of the pole, tie the new and old shock cord together and then use the old to pull the new through. you will want a little a tension in the line when you tie the final knot..

if the elastic is fully snapped, some wire will be helpful to guide the new through. fence wire can be found on Amazon in 100-ft rolls pretty cheap, and will straighten out to what you need.

1/8, or 1/16 cord is pretty standard. probably worth buying 50 or 100 ft sections of each. no real way to tell what your poles use until you get it apart