You should be able to get them anywhere you can get the other ballasts, basically any industrial lighting supplier.
If you want to do some reading (you would not believe how complicated these ballasts can get), try this site:
www.advancetransformer.com (go to "Catalog" then "HID Lookup")
You should be able enter information such as ANSI code (i.e., M137 or M57 or whatever), or "MH" and "400W" and so on, then press search, and you should get a list of specs (and more importantly, part numbers, which is handy when you order at a lighting supplier).
So for example if I was interested in a 175W metal halide ballast, you enter iin that info, press search, and get 17 results. For our purposes we want the "quad tap"(*) (which probably the most universal ballast), so the part # for that is "71A5592."
(*) - Quad tap means there are 4 taps. By "tap" that means what power supply type to use. Household electricity is 110V or 120V or whatever so that's the tap you'd want to use, but if you had 220V you could use the 220V tap, and so on.
So anyhow armed with this information you can go to a lighting supplier and ask for a "metal halide ballast made by Advance Transformer, part #71A5592 with long range ignitor" and then they should be able to look up a price for you.
There's enough info on that website to make you dizzy. Looks like you can actually take a course on metal halide ballasts!
PS. In a pinch, the probe start ballasts are not the end of the world. But I thought I would mention the pulse-starts because I wish someone had made me think about this before I had bought the probe start ballasts. I run 2x175W Ushios on one tank, and Ushios run better on pulse starts. Sometimes my lights don't fire up right away, it can take anywhere from 1 to 10 minutes for the lamp to strike and stay lit. If I were running pulse-starts I wouldn't have that problem. But, since I made the investment already, I choose to live with the phenomenon since the money to replace the ballasts could be used to buy other toys... er ... "tank stuff" in the meantime.
I hope I haven't muddled the issue too badly for anyone!