sliding window guide

This looks like the track piece on a casement hinge set like the 36-004-10. Unfortunately, casement hinge parts weren't built to be replaced separately; when one piece breaks, it's a sign that the entire hinge set needs to be replaced. See if the 36-004-10 matches what you need. If this in fact a part from a different piece of hardware, please show us the assembly and we'll do our best to find what you need.