Does anybody know where I could find a good affordable 400w mercury lamp ballast that I could use to run a 400w blacklight mercury lamp?

Go FB market place and keep eye out for high bays lights
 Those  are getting scrapped by the day

However, the only difference that I see with the probe start metal halide ballasts is the fact that the probe start metal halide ballasts have a higher secondary voltage (OCV) compared to most mercury vapor ballasts made before 2008 with the exception of 1000W H36 mercury vapor ballasts.

Would you be okay with using this 400W H33 mercury vapor ballast linked below?:

https://www.ebay.com/itm/384798739673

It only costs a little under $32 with free shipping, but it is a proper 400W H33 mercury vapor ballast if you prefer to have a proper ballast. Like others said, you could also look for some 400W M59 probe start metal halide ballasts as well since they are more common and can still run 400W H33 mercury vapor lamps with no problems.

Cheapest would be to find a 400w M59 MH highbay on CL/FB Marketplace and take the ballast out from that. Most older M59 ballasts are also rated for H33 lamps. Prices range from free to $20 depending on what you find.

However that H33 ballast linked above is one of the cheaper ones on eBay since it also includes free shipping.