I am looking for a ballast for a 100 watt mercury vapor lamp.

A MH ballast with a suitable rating is totally fine, as long as it won’t self destruct or blow up the lamp. Also at a reasonable price.

Thank you.

  U can take a 70 watt MH ballast to run 100 watt MV lamp.  Just daconnect the ignitor.

Oh perfect, thanks!

Yeah, I usually use 70w MH ballasts with the ignitor disabled to run 100w MV. You can actually keep the ignitor wired in but with a switch to break either X3 or X2 which will prevent it from firing. I have a setup like that in my R47 to allow me to quickly change between MV and MH.

70w MH ballasts seem to be around $30 or so on eBay with free shipping. There's also a seller with NOS 100w MV H38 kits for around $65 if you prefer that instead.