A few pre-Intel Macs like the first few iMacs are USB-bootable but they are in the minority. Most PowerPC Macs desktops need to boot from a FireWire device. One member may return here to post his successful experiences with making non-USB boot Macs boot from USB, but feedback left by others show the results of his procesdure are often spotty. If interested, he has posted instructions on Youtube. Your mileage may vary.
There are some good external optical drives on the used/pulled market that are Mac Bootable (not all are). I have a Sony DRX-820UL-T external DVD combo drive that has FireWire 400 and USB and has proven Mac-bootable as far back as a circa 2000 iMac G3. I picked it up with most accessories for US$5 at a rummage sale. This article has most of the details:
http://www.creativemac.com/article/Sony-DRX-820UL-T-Review-48394
Here are some on eBay:
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0. XSony+DRX-820UL-T.TRS0&_nkw=Sony+DRX-82…
Please note that I can only vouch for the exact model i have. Others may not have FireWire connectivity or may not be Mac Bootable.