Jettas came with two master keys, two "banjo" wireless key fobs, and one valet key. You can purchase a new or used banjo online (eBay is your best bet for both). Programming a remote is simple, even if it was previously programmed to operate another Jetta.
Note that 1998 Jettas came with two different types of banjos which look identical but are not interchangeable. I think the difference might be between US and Canadian models, but I am not sure. Luckily, I also have a 1998 VW Jetta TDI which originated from a dealer in Florida, so chances are you need to look for the same banjo type that I have, which has "FCC ID: M3GHU01WT CLARION" engraved on the back. The other type has something else engraved on the back. Most eBay retailers will give this number to you as a "part number." The link schultp gave you has a banjo with the correct number. I wasn't able to verify the one on the link you gave.
Here's the programming instructions that worked for me.
1) Insert one key into the ignition and turn to the "on" position, i.e. the position you would have it in while driving. Note that the engine does not need to be running.
2) With the other key, lock the driver's door, and hold it in the lock position (about 5 seconds) until the horn beeps three times. This puts the car in the programming mode. Any remotes that were previously programmed with the car will be erased from the memory.
3) Press any button on all the remotes you would like to work with this car (one at a time). You will hear one horn beep for each remote the car recognizes. You can program up to four remotes to work with one car.
4) Turn and hold the key in the lock position again until you hear three beeps. This exits the programming mode. You may now unlock the car and remove both keys. To verify, test all of the remotes.
You can program one remote to work with as many cars as you like.
