Ammonium Chloride is much easier to get than Ammonium Sulfate and is also used to speed up Sodium Thiosulfate based fixers. At higher concentrations the Chloride forms soluble Silver complexes, which means the Chloride ions don't hurt the fixer as some might expect. Look for Agfa 304 or this recipe for a sample composition. You can add 10 g/l Na2CO3 to Agfa 304 to get its pH up to about 6. Don't go much beyond pH 6 or the Ammonia smell will be obnoxious.

If you are unsure whether a Sodium Thiosulfate based fixer is strong enough for archival fixing, use retained silver tests to find issues, then use two bath fixing to solve them.