I don't see why store bought apples wouldn't work quite well. The charcteristics of apples vary from year to year but are generally similar. For example, I have figures for the RD and GS from a few years ago at RD SG1.053, pH 4.3, TA 2.6 and GS 1.046, pH 3.5, TA 9.2... not dissimilar to the 2023...