There is! But there aren't any signs to tell you...
As explained on the PTV Airport Buses page, the SkyBus ticket office at the airport sells the myki Visitor Value Pack. That costs $14, and comes with $8 credit (plus a few other bits).
There are no signs at the SkyBus ticket office about myki, it isn't listed on any of the prices shown, but if you queue up and ask they'll get one out of a secret drawer and sell one to you!
Once you've bought your myki, it's a moderately well signposted 1 minute walk to where the 901 bus goes from - follow the signs for Public Buses.
Alternately, there are in theory some regular myki topup and purchase machines in T2, T3 and T4, but they're not well signed and rather hidden. You're looking for a silver/grey machine with a blue sign on the top. I couldn't find the T2 one, the T3 one is on the far left as you go to leave the baggage claim area by the toilets. For topping up a myki it may be worth hunting, though don't forget you can also top up on the bus! For visitors, heading to the SkyBus ticket office to buy one is likely to be much quicker and easier