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

I have on that looks very similar to this, along with the papers.  Papers are important, since there are many fakes out there, and the papers can be authenticated with the Vatican.  Father Jenkins looked the ones I had and vouched for the papers, saying that he knew the Cardinal who had signed them.  Father Jenkins knew lots of people.    But, as I said, if you go through the Conciliars, even though they're no longer Catholic, you could certainly get a verified genuine, even if the papers bear the name of a "Cardinal" Pedophile.

That's a nice one. I just checked eBay and it's outrageous some guy wants $5,000 for one! 

I will say that normally lay people should not have this relic 

I totally agree.  In the past this relic was reserved to churches and only VERY RARELY did a lay person acquire one from a priest relative etc.  Lay people are too busy with their state of life to accord it the reverential treatment it deserves.  When the occasion called for it the priest could bring it to the lay person but usually the priest brought the Blessed Sacrament.
I've known people who say their parent/grandparent had the relic of the True Cross and none of them knew where it was or what happed to it.
Some things belong in the church only.