Day night shade replacement suggestions?

What size line, and can you recommend a brand? I usually have to restring my blinds annually...

I used 60-70 pound test braided/teflon covered fishing line. A big roll at BassPro is under $15 and will do dozens of these. It is very strong, slides easily, and will resist the fraying at the joints that messes up the strings.
There's several online videos of how to do it. Fussy, but once you do one, the rest are easy.