So ... Ebay is generally not the right place to look, as it's not really permitted to sell them. Yet ... many such relics have gotten into the hands of infidels / pagans / heretics when many churches closed down after Vatican II and the faithless clergy dumped these treasures in various fire sales. I knew a furniture shop in Chicago that had tons of relics, sacred vessels (including chalices), church-quality statues, art, etc. It's permitted to buy them from such scuм and villainy in order to restore them to Catholic use. It's like paying a ransom. So, with all that said, buying on Ebay might be licit.
But, if you're willing to go through the Novus Ordo, it's possible to write to Rome (it's customary to offer a donation for the reliquary) to request a tiny sliver of the True Cross. I used to know whom to write, but I don't have that information anymore ... but I think if she were to call the office / chancery of the Conciliar bishop in her area, his office could probably get that information for her.
Of course, today's Vatican sent out a couple fragments of the True Cross to the heretic usurper Charles of England for his "coronation".
https://www.usccb.org/news/2023/vatican-sends-relic-true-cross-britains-king-charles