Any of the female to female DB9 cables listed here will do the job.
http://www.cablewholesale.com/catalo...rialcables.htm
This is a 25 foot version for $4.39.
http://www.cablewholesale.com/specs/10d1-03425.htm
25' tethers are common and normal recommendations would be to stay under 50'. I've been able to run long tether cables up to 100 ft without a problem, but mileage may vary. Over 100' you need to power the OI separately, and eventually the signal degrades with too much distance.
P.S. I've used converters to go from serial to Cat5 and back again for long distances, but the OI must be powered separately, and the converters are far more expensive than a $4 serial cable. They just happened to be available to test with.