I've bought line from The Chandlery in Ottawa - mail ordered at resonable cost.
Reflective line is harder to find. Lawson Glowire is often mentioned online for guyline diameters; they also make 5mm. However, whenever I've looked it seems to be out of stock.
Rigging my boats these days, I use Amsteel (7/64", 1600# break strength) inside 1/4" black plastic irrigation tubing.
Larger tubing over standard deck lines makes them much easier to grab, too.
I've used quite a lot of Samson Trophy braid (black) in both 1/4" and 3/16", too.
One idea to consider: Make your perimeter lines a different colour from the shock cords (and rudder lines if they are above-deck), so that swimmers and folks you're assisting in re-entry won't 'grab the wrong one'.