Common names down here include 'buffer' for 'alkalinity up' (aka baking soda). Most bags of CYA I've seen are called 'stabiliser'.
Adding lots of baking soda is part of the pH roller coaster promoted by pool shops, which keeps them selling hydrochloric acid and buffer, over and over and over. (acid reduces TA so you add buffer, high TA needs more acid, rinse and repeat). If your friend uses chlorine tablets, then buffer is needed to counteract the acidifying effect of the tabs, but otherwise, buffer is almost never needed. Cheapest source is usually the bicarb in the baking aisle at Coles. The pool industry recommendation here is 80 to 120 TA, and one pool shop near me recommends 180! Around 60 is usually good for TFPC, but could be a bit low if using trichlor tablets to provide chlorine.
The chains like poolwerx are sometimes the most expensive and like to throw in some high pressure sales with a visit. An independent pool maintenance company that has been around a while would be best in my opinion. They make most of their living maintaining pools, and usually the equipment changes are their gravy. They keep everything they need with them so they can be in and out quickly for a change like you're considering. I would just watch for an independent in your area, or use google reviews. Your PB might know a good independent. They don't need to be a qualified plumber to do pool plumbing, except if there's an attachment to mains (e.g. for an autofill) or an attachment to sewer (e.g. a backwash line), so the poolies do the pad plumbing, repairs, replacements as a sideline for customers.
If I had cartridge or DE filter, I'd see that differently.
