We know that Starman has had some adventures travelling through the Solar System. Is it possible for us to visit Starman? The way that engineers determine this kind of thing is with a porkchop plot, as seen below. This uses the same set of data that is loaded into the website, and thus will only show possible launches for the next few years.
These numbers can then be passed into NASA’s Launch Vehicle Performance Website by placing the C3 value (Which is the square of the delta-v value). For example, when I first set this page up there was a launch opportunity for a flyby of Starman, launching March 2028 with a 224 day mission and a delta-v of $4.276 km/s$. That translates to a C3 of $18.28 km^2/m^2$. This will tell me all of the launch vehicles that I can use, along with their projected payloads, to make the mission happen.