They're sold in pairs. You need the width if your design fabric or a little longer. With that, you need extender bars & 4 knobs. Extenders not to long or you lose tension on the sides of the fabric.

Don't buy stretchers with split rails like JoAnns sells. They don't hold. Some like American Dream's EZ Stitch system. Since you're used to Q Snap, there's a other company that uses those C clips. Forgot the name. An internet search should turn up a lot.

The scroll frame that use the C clamps is Handi Clamps by F. A. Edmunds. It is the one that I use and I like them very much. But I don't think they are considered stretcher bars, this is a scroll frame system.

Stretcher bars are used for needlepoint and counted canvas work, not cross-stitch. The fabric is tacked on to the bars to make it very stable while stitching. You use a set of 4 bars for each project....one on each side of the fabric, so the bars need to be close to the size of your fabric, or a little smaller. They lock into each other in the corners. If they are too big, you won't be able to pull the fabric far enough over to tack it.
If you are looking for 'bars' to do cross-stitch, then what you want is a scroll frame.

True. So many terms! The fabric for cross stitch would likely sag or pull out of the tacks anyway. Scroll frames usually come in complete sets, or pairs of rods, bars, packs of 4 knobs or whatever.