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
travel.stackexchange.com/users/26/Gagravarr
Thought I'd provide an update since these posts are all ~4 years old.
There's no sign of a Myki station between Qantas domestic arrivals and and bus terminal (about a 5 minute walk).
You can, however, purchase a Myki for $15 from any of the Skybus ticket kiosks.
This includes $9 of credit and is ready to use immediately.
travel.stackexchange.com/users/89687/Emmylou Cooke
You can actually buy a ticket on the bus itself.
I've done it, and as you can see on the official website, it is one of the published official outlets.
From: https://www.ptv.vic.gov.au/tickets/myki/buy-a-myki-and-top-up/where-to-buy-and-top-up/
Where you can buy a myki You can buy a myki At over 800 retail outlets including all 7-Eleven stores and selectedretailers where you see the myki sign the ticket office window at Premium Stations online
(link opens in a new window) by calling 1800 800 007
6am - midnight daily from a myki machine (full fare myki only) at all metropolitan trainstations and V/Line commuter service train stations and selectedaccessible tram stops and bus interchanges PTV Hub
onboard buses
EDIT: I'm not aware of any restrictions with regards to purchasing myki tickets on a bus.
My understanding is that you can purchase any kind of ticket on any of the standard metropolitan buses.
I.e - if the bus is not a regional (V-LINE) or charter-bus, you should be able to purchase Adult, Concession and Senior myki tickets.
The tickets are differentiated with a single letter on one side and by the amount charged per journey.
travel.stackexchange.com/users/23145/enhzflep
There's also a vending machine within Melbourne airport where you can buy (and presumably top up) a myki.
travel.stackexchange.com/users/324/Andrew Grimm
An alternative not mentioned in @Gagravarr's answer is that you can purchase a myki online from the PTV website.
travel.stackexchange.com/users/6811/Mark Micallef
I've had a read of the  Wikivoyage page for Melbourne airport , and discovered the much much cheaper way into the city via the 901 bus to Broadmeadows then the train.  However, in order to do this route, I really need to get myself a  myki smart card . Looking on the  myki website  at the list of retailers and stations you can get one, there's no-where showing at/near the airport.  Is there somewhere not listed on the myki website which sells them at the airport?
Is there anywhere at Melbourne Airport to buy a Myki?
travel.stackexchange.com
<public-transport><tickets><melbourne><mel>
travel.stackexchange.com/questions/39563
travel.stackexchange.com/users/26/Gagravarr
You can buy a myki online, on the bus itself or at the Skybus ticket office, although this is not indicated at the ticket office itself. On arrival at Melbourne airport, a Myki can be bought at the SkyBus kiosque, from a vending machine or on the buses. Before arrival, Myki can be purchased online on the PVT website. 
A Myki can be bought at the SkyBus kiosque even though there is no sign. Tickets can be bought on the buses. There is a vending machine at Melbourne airport selling Myki. Myki can be bought online on the PTV website.