Measure the lens and either go to Adorama or BH Photo websites… Most (read as last time I bought them… Ages ago) fit standard lens filters or you can buy a “step up” ring to make larger lenses fit smaller Barlow’s.
Why do I want to buy a BIGGER protection lens? Hey… Glad you asked! It’s simple- a larger lens will allow more light into the Barlow and prevent “vingette” effects along the edges of your field if view.
Hope this helps!