The lens didn't come with one?
If I'm thinking of the right lens, it had a 62mm front filter, and came with a 'petal' type hood (deeper sections at top/bottom, shallower at the sides).
You could try ebay or camera shop bargain bins for one matching that, or a standard 62mm round hood would be fine too - as long as it wasn't deep enough to cause shadows at 28mm.
Or, the Cony/Carl Zeiss 16-80 is a 62mm fitting too - it comes with a petal shaped hood that would be short enough to not cause a problem at the wide end (though give minimal effect at 135mm) - it uses a " ALC-SH0005 " Sony lens hood, available through sonystyle.com or retailers
Similarly the ALC-SH0008 - designed for the Sony 18-200 would be compatible - it's also 62mm fitting and would be fine for both the wide end and long end of your lens range
Hope that helps