Well,
Ebay may be your best bet, everywhere else may have much higher prices.
For the CD drive,
As far as I know, I don't think that they make a DVD-RW drive for an iMac g3, or a CD-RW and DVD combo (someone else may correct me if i'm wrong)
As for prices, i searched ebay and for a CDRW it was more expensive than i expected, i didn't see much but most of them were $49.99 plus $12.99 shipping.
If you want DVD i found this on ebay ending in 6 days, it is $24.99 plus $9.99 shipping and it claims to be tested.
the iMac g3 uses an IDE hard drive. If you have an older PC, it most likely uses an IDE hard drive, so if you have an extra thats big enough you can put it in there. The iMac g3 will support up to 128gb without third party support. Again you could buy from ebay, or maybe try tigerdirect.com or geeks.com may have them. Expect to pay from $50-125 depending on size and what not.
You could also buy a processor upgrade card. I have never used one of these and i really don't know much about them at all. You would still be able to use OS X with most of them. Try doing a google search and you can find more info.
You might also want to upgrade your VRAM if at all possible,
Look under about this mac and see how much you have,
Do a google search, VRAM is so cheap, you may want to max it out, the limit is either 8 or 6, you can also find that out on google. Its worth the extra $20 or so.
Also,
I've heard it is a royal pain to take apart the case,
So here is a guide
http://www.scribd.com/doc/103447/iMa...assembly-Guide
Towards the right you can download the file.
Hope everything works out,
and we hope to see your results.
Tanner.