i made mine from some cheap, small, walmart bungi cords. they came about 10 to the bag, and cost under $3. they're about 8-9" long including the hooks, maybe 7 or so for just the elastic part.
you take one hook completely off the bungi. a loop of elastic with a staple around it is what's left. run the end of your tieout cord throught that loop, then tie an overhand on a bight, leaving about 6"-8". tie that bit to the tarp. about a foot or so down the cord from where the bungi's knotted in, tie another overhand on a bight. the remaining hook from the bungi can tie in there. you can move the hook or the knot, for whatever works best for you. you can even tie a series of knots, enabling different levels of tension.
i need to get a picture of these things posted... would save me writing (and more importantly, you all reading) so much.