This really is a problem but for now either buying it from a friend who got his hands on some or trying it with a DEX.
Discussion
Does Digital Signature prove Bitcoin Ownership?
yes
But if these digital signatures are all visible/available on blockchain then how does a person know which of these public keys is linked to his/her own private keys?
You see addresses on the blockchain not your digital signatures. You use your digital signatures to authorize a transaction.
You can track addresses but their is no way of knowing who's addresses is who unless they publicly shared their wallet (addresses). That's called pseudo-anonymous.