First, it's very easy to replace the soft top. All you need is a phillips head screwdriver and 10-15 minutes max. The front is held to the header with a half-dozen phillips head screws, the rear by velcro. Second, a used factory soft top isn't normally hard to find in the various Jeep oriented For Sale forums or Craigslist. Try to find one from a 2000 or newer because that will be made from the multi-layer Sailcloth fabric as made by Bestop.
What'd I'd recommend (and did myself) is go with Bestop's Sailcloth replace-a-top. Sailcloth is something like 3x thicker than the top your '98 and my '97 came from and that benefits you with a far quieter interior from significantly reduced top flapping and decreased road noise in general. The reduced noise level of Bestop's Sailcloth soft top is dramatic, I noticed it immediately after installing my Sailcloth top.
I can't say the cheapest place to buy one however, I bought mine used.