The sticker on the first photo shows what kind of lamps you can use.

From what I can see, it can use either a M59 lamp or a H33 lamp.

If you google those, a M59 is a 400W MH bulb and a H33 is a mercury bulb.

So, you just might be in luck.

Here is a link to 1 place that sells those lamps:
M400U - hid - metal halide light bulb lamp - donsbulbs - light bulbs lamps valve valves tube tubes

Try your local electric supply / industrial lighting shops in your city. You might be able to get it cheaper without paying shipping.
Hempfester Reviewed by Hempfester on . help with this ballast please i have this old ballast, i'm not sure if it's mh or hps, the guy i got it from said he thought maybe it was a mercury vapor, i'm hoping not though... it's a white box with a big loop bold coming out of one side and a large socket coming out of the other, i took it apart and can see that it was originally 277v, but it has wires to switch to 208 and 120v. someone has already rewired it for 120. there's a sticker on the inside saying hpf, 400w, it also says ruud lighting. i'll upload pics soon as Rating: 5