If you were going to use it immediately then yes you could use sulphite. Metabisulphite in an alkali solution forms sulphite anyway but it's needed in Part A to prevent oxidaion during storage.

What's noticeable is that when Part A collapses the Metabisulphite has stopped giving off SO2 which is the anti-oxidant and broken down to sulphites. It's important to use reasonably fresh Matabisulphite rather than Bisulphite which can be a mixture of Metabisulphite & Sulphite.

Metabisulphite is very easy to find as it's used in the food industry and winemaking etc, so anywhere selling supplies for home wine making/brewing will have it and it's inexpensive.

Ian