Dithane 945 contains Mancozeb - this is the active ingredient, the 945 bit is just the trade name! you can get it from garden centres. Its a protective fungicide so won't cure the rust, but will certainly prevent it getting worse.
My garlic suffered with quite a bit of rust last year, i didn't spray them and the yield was quite poor. I also think they suffered with a bit of fusarium basal rot (which i mistook for white rot originally
thank god it wasn't!) which won't have helped.
Bizzare as it might seem you could try spraying with a garlic spray (New Horizon is a brand i've seen in garden centres), as allicin (in garlic) has fungicidal properties and is 'natural'.