X2 on getting Crosby Shackles!!!! They are expensive, but are properly made, identified (load rating and makers marks) and proof tested. Any commercial rigging shop will have them (look up Wire Rope in your yellow pages).
Beware anything made in China, they are really common in farm shops and hardware stores, almost always of poor quality (but pretty!), dubious material and are not usually adequately proof tested (ie, they could bend or shatter under load, and really leave you in a whole world of pain....). I have actually seen them make shackles and pad eyes out of cast iron or steel, with no subseqent heat treating... very weak, VERY brittle.... Ka-Boom!!
There are other good brands of rigging than Crosby too, some american, some european, check with your local professional suppliers to what they have, and use, and what certification it has.