You can find a number of packages with these sorts of shapes in them in various digiscrap stores, or you can make your own. The following instructions help you make a scalloped square that you can use to clip your paper to, as Cathy suggested.
Start out by making a new shape (like a rounded rectangle) on a new layer, and rasterize that layer if necessary. If in PS or PSE, control-click (command-click on the Mac) on this layer's thumbnail; that selects the whole shape you just made. Then go to the paths panel and choose "Make Work Path" from the drop-down menu.
Go back to the layers panel and add a new layer on top of the shape layer. Choose a brush that will give you the desired size of scallop (say 100 pixels, 100% hard) and set its spacing to 100% and no scatter or other dynamics.
Return to the Paths panel and choose "stroke path" from the drop-down menu and reply "with brush" in the dialog box. Check the result, as it may give you a slight overlap in the start area (the top left corner, if you started your square there). You may have to then undo and adjust the brush's spacing by a few percent + or - to get just the right join. Once you have that done, you can delete the work path and merge the two layers.
If you want to get really creative, you can (after you merge the first two layers, and before you delete the work path) stroke the path again on a new layer with another brush like a smaller round hard brush, or a heart brush or star brush. It will be necessary to play with the spacing on this second stroking to get it to line up within each of the existing scallops, so be sure to use a different color for the second stroking so you can see it on top. Once it's to your liking, control-click on the second stroke layer's thumbnail, switch to the merged layer, and hit delete. That makes a hole along the scalloped edge. Then delete the work path and your clipping mask is ready!