You can't buy it yet, but the technology is here, and now it's a matter of when this 5G technology will become mainstream.
We're essentially waiting for ISPs to begin rolling out 5G. At the moment, Verizon and AT&T have both announced that they'll roll out 5G wireless internet later this year.
So far, no smartphones support 5G because there aren't any mainstream 5G networks to connect to. Once 5G networks begin rolling out later this year, we'll surely see smartphones with 5G support.
As for home internet, there's no indication of how much 5G routers and other equipment will cost, or how much those internet plans will cost. You can get a 500Mbps plan from Verizon Fios at the moment, but that costs $275 per month.
Here's hoping gigabit 5G internet will be cheaper.