The story that officers in the military by WW2 had to purchase their own uniforms is literally true, but almost all references to this custom seem to omit fact that such officers on commissioning were paid, as of right, an Initial Outfit Grant (courtesy of the taxpayer) which was allegedly sufficient to purchase all the required items. However in real life it never was, so the disgruntled officer had to gripe about making up this shortfall. Particularly impecunious officers also had the option of purchasing second hand uniforms (usually had to be slightly re-tailored) from "the trade" and at some time during WW2 the Air Ministry even ordered (in bulk) officer uniforms in standardised sizes (like an airman!) which could be purchased (I think) at RAF clothing stores to keep prices down, using the same grant. The methods of obtaining and maintaining uniforms has evolved over the years, and in some cases (and services) an annual maintenance of uniform grant is paid to all ranks for this purpose (after the initial issue).
David D