I think what you are looking for is a turnbuckle. You can get them at Lowes, Home Depot, Grainger, or McMaster Carr to name a few. Most general hardware stores probably carry them too.
You may be thinking of an 80/20 assembly no. 4051 (180°) or 4139 (90°) or similar, which can be found under "dynamic pivots" in their catalog https://www.8020.net/Catalog-Intro.asp These pivots are different than the turnbuckle angle adjuster JAllen mentions - the turnbuckle is stouter than the 80/20 pivot clamps, but you still need some kind of pivot. There are a jilllion 80/20 distributors.