Hi Christopher,

it depends:
psi is a Greek character (U03A8 for uppercase), so it is included in every font with Greek language support.

Then, it could be used as a symbol in mathematical typesetting. There are a couple of codepoints, depending on its application: U1D7A7, U1D6BF, U167A7, U1D76D, U1D733, U1D6F9.

Special fonts like Cambria Math will likely contain these glyphs.
F

Can you recommend actual font names? I don't quite understand your response.

Oh, I’m sorry I confused you with numbers.
Basically, I wanted to ask you whether you’re typesetting Greek text, or if you just need the ‘Ψ’, e.g. for a mathematical application.
If the latter is true, then what is your basic typeface? Regarding style/weight, it better should be a close match, no?

It is for a poster. I would like to have as many Psi characters on it as possible.

Ah, okay!

Here are a couple of fonts with Greek language support distributed by FontShop (check out ‘Character Set’ to view the ‘Ψ’), and some more by Linotype (» ‘Encoding map’).
I couldn’t figure out how to filter Greek fonts at MyFonts.com in a reasonable way.

Luc Devroye maintains a List of Greek fonts.