By plastic straps, I believe they mean wire ties, aka zip ties. I have heard that its ok to drill holes in live rock to loop the wire ties through. This sounds kinda tricky to me (I am about to order about 90lbs. of live rock next week for a new tank), as I will be setting up my pile o' rocks in the tank, then to have to take them out, drill holes in strategic places so that I can put it back into the tank in exactly the same pile that I had originally and run wire ties through little holes...
This sounds way tedious to me, but maybe I'm making this out to be more difficult than it really is.
To make a long story even longer, to answer your question, I believe "plastic straps" are wire (zip) ties. You can get a ginormous bag of assorted sizes and colors at Lowe's/HD for a couple of dollars. Good luck.