I've had good luck using the plastic beads that kite flyers use for the strings on kites. Kite cord is plenty strong for most tent and tarp applications.

Nice thing about the plastic cord tensioners is that they stay put. Once adjusted they stay that way until you readjust them. So when I take the tent or tarp out of the stuff sack I know they are the way I adjusted them last time I used them.

I see this link also has a small aluminum cord tightener. Looks good but I haven't tried it.
http://www.kitebuilder.com/catalog/index.php/cPath/25_42?osCsid=eebd4eec967a20811e83ee474d078a8c

PS I forgot to add that the plastic tensioning bead is kind of difficult to put on the cord. It is something you would want to do at home and then leave it alone.