Newegg has the usb->ps/2 type converters for $5.00 each.
When you say you need a serial ports do you mean the old rs-232c type? Newegg also has usb-rs232c converters for around $12-13. I actually had no idea they made something like that until I did a search on the site, there are actually quite a few vendors that make something like that. No idea how well they work, but you can find rs-232c cards pretty easily/cheap (Newegg has those also). I think I would go for the card instead of the adapter, those usb cable coverters seem a bit iffy (I would think there would be voltage issues?).