I have this exact camera, except it has 'OneStep 600' written where yours says 'Spirit'

I'm in the same boat, you gotta track down disposable flash bars made by GE, Sylvania, or (rarely and in my case) Polaroid themselves. I have only seen Polaroid branded ones once, and I bought all three that were on sale. The most common that I've seen is Sylvania.

Your only alternative is hold the camera in one hand, and hold a bright light source or some sort of exernal flash gun in the other (or get a friend to hold it for you).

Polaroid could, and should, make more disposable flash 600 bars, or a resuable one (like the ones made for the SX-70), but they haven't indicated that they will as of yet. 'Till then, my advice is to buy these up when you can. Because as years go by, these will become harder to get your hands on.