It also seems to be available from Rowlands in Cardiff
Be sure to type “Grace Williams” in the search box, because Hiraeth is a general Welsh word meaning a nostalgic longing, and there are a number of pieces by that name. The lady who runs Rowlands is herself a Welsh harpist, so she will be pretty clued up. I do see the Hiraeth you’re after listed in her catalogue so you should be in luck.
Grace Williams was a Welsh composer, so the people there are bound to be able to help. If the above fails, try: