I need to alter some knit tops. I am noticing that the shoulder seams have a transparent plastic strip sewn in with the seam. It is not fusible but works excellent as a stablizer to keep the seams from stretching too much.But it can stretch a little.

There are no places to buy sewing supplies here. They went out of business years ago. I can't find an internet supplier of a roll of these strips.

Can buy plenty of patterns and knit material online; but no one mentions these plactic strips even though ready-to-wear seems to have them is almost all their sewn knit tops.

Would love some info. Tks.