It's not hard to make one out of thin card board or clear plastic from a takeout container's lid. Try it out and see if it is helpful to you. If it is helpful, and you'll be using a LOT a purchased one made out of durable plastic may be worth it to you.
I've done Y-seams a few times, and I've just marked a line 1/4" over from the edges of the fabric for the last inch, on each piece of fabric involved. Where the lines intersect is where you want to stop sewing. (See this
video at 0:37 for a nice illustration of the lines intersecting.)