I have a laser like you describe on my machine. You can take any design --- let's say for a motif (12" square) and lay it on the quilt on either side of your machine. I hook the laser up to a spindle on the top, plug it into the machine in the back, then I line the laser up with the pattern -- making sure the design will be sewn exactly where I want it. I can then start the machine and watching the laser light on the pattern, quilt that design.
If you have the ability to quilt from the back of your machine, you can lay multiple designs down on the table and do the same thing......helpful with edge to edge designs or borders.
That being said --- I've only used mine once.....I'm just not that steady with it. I know there are some people that use theirs a LOT.
Mine came with my machine....I can't help you with locating them. I have the Handiquilter Avante.
I hope that answeres your question.