Any of the manufacturers, that manufacture to accepted standards, such as Crosby, Columbus Mckinnon, Chicago, Titan, Campbell, and for stainless steel
, Suncore make chain or connectors that meet the standards for dimensions and strength (WLLs). If dimension or strength is important for your use, choose a manufacturer that follows the standards. For chain, it will be the standards established by NACM or by ISO. For connectors the standard, in the USA, is Fed Specs-RR-C-271E.
For shackles, to meet the standard, the information on the shackle, such as WLL and size, must be embossed, in other words this information must be in raised letters and numbers. If this information is stamped in, then the WLL and size will be as stamped, but the dimensions may be different than that specified in the specs, often bigger. If you are looking to join to chain, it is best to pick connectors made to the specs as these connectors are the specs are used to assure a match.
Keep in mind that you can match "low-strength" connectors to Grade 30 chain, but for Grade 40 chain, you must use "high-strength" connectors, if the WLLs and dimensions are to match.