I see Libra facebook coins already registered on the coinmarketcap site, but data about where the market and exchanges do not yet exist so it is very difficult to find Libra trading, only one option is there, that is, patiently waiting to buy, because the price does not yet exist.
I would be surprised if Facebook doesn't gift it's billions of users a token amount of Libra to introduce everyone to it's money system. Facebook has obviously decided it wants to learn more about everyone's spending habits and click bait advertising hasn't done them any favours over the years, so why not just start their own crypto currency and sit back and watch what each and every user does with the funds given to them at start up.
I'm sure the Libra will either be pegged to a fiat currency, or it will be sold like gift vouchers in ten, twenty, fifty or even one hundred dollar increments.