The short answer to your first question is yes, and yes. There are catches though. To see detailed answers go to xlr8yourmac.com's G4 Systems page, especially the FAQ which is the first link, and their CPU upgrades page.
In short, ones from newer (133MHz bus) G4s can be used but may need modifications to either the CPU module, heatsink, or Mac.
http://eshop.macsales.com will give you the best price on new CPU upgrades. If you go through their "select machine type" menu it will only show ones that are guaranteed compatible with your machine, so there's no confusion.