M57 ballasts you could probably still get from lighting stores or eBay. H39 ballasts you'll have to get NOS from eBay. If you're in the US or Canada you could also try going to your local Habitat Restore and seeing if there's any ballasts or fixtures that can run 175w lamps. 175w MV was a very common wattage so you might be able to get a used fixture there.
I once saw a 175 watt light at a Restore but I went back the next day to get it and it was gone because where I live, they are super popular. I plan to check tomorrow to see if I can find one but for now I am temporarily running it on a 250 watt hps ballast (why I originally posted my question) and the ballast it in 1 of 2 street lights I found at the Restore. It works well but when it is at full brightness, it flashes brighter for a few seconds so that's why I am looking for a dedicated fixture for my bulb.