around here any reputable farm supply store keeps racks of different size and shapes of shackles.
not knowing your application or where you be from that's best I can offer.
I just bought some galvanized ones yesterday and came from a place called Princess Auto which is on Cdn side of the border. Normally I would buy from Farmers Supply which is on the U.S. side of the border..
here are various std. sizing for you though.
Also these stores will have
bushings in these sizes if you want to use smaller clevises. A shackle in the size I think you are talking about maybe have working load limit WLL in the 7 - 15 tonne range! I am going to guess a bushing to size down the shackle size maybe something you can consider?