TurboTax for 1998 is no longer available. Even if you could find a copy, it probably would not run under any current Windows or Mac version. And even if it did run, it would likely be a preliminary version, without the final updates, and there is no way to update it.
If you just need to see a copy of the tax return, you could try using TaxPrinter. You send them your .tax file and they send you back a PDF of the tax return, which you can then view or print with Acrobat Reader. But the FAQ on the TaxPrinter web site says "TurboTax 1998 and 2002 will only print the tax summary." TaxPrinter has no connection with TurboTax. I have never used TaxPrinter, so I cannot make a recommendation. I'm just making you aware of it.
EDIT: Revised statement about TurboTax 1998 from the TaxPrinter web site.
