educovas
macrumors regular
Unless you look only for Quad or 2.7 DP, everything else is regularly available at very low prices, and in the US perhaps often for free. (But shipping may involve insane costs, so it depends.)
Or, given that you are developing for the platform, maybe ask here in a dedicated thread or/and on Reddit if someone in your geographical vicinity may donate a PowerMac?
I live in Brazil, computers selling for low prices are almost impossible to find. PowerPC Macs are kinda rare here now so they are also expensive.