transformer for DI Box

[Trent]

I'm planning on building this DI box www. freeinfosociety.com/electronics/schematics/audio/cb1directbox.pdf, which is super super simple. It just changes a something's output from a high impedence instrument level to a low impedence microphone level. I'm really new to electronics and the problem is I don't know what transformer to buy in order to do this. I know some amount of stuff about kind transformer I need, but I don't even know where to start looking to find one suitable. If any body could help me with a manufacturer and model number or something, that would be really great.

Thanks