Your best bet, if you're determined to have a 1998-ish Buick Regal with a manual transmission, would be to:
* Consider other years: Research Buick Regal models from earlier years (pre-1997) – there's a *slightly* better chance of finding a manual transmission in those, but it's still a very uncommon option.
* Check specialty car sites: Websites specializing in classic or used cars (e.g., Hemmings, Bring a Trailer, etc.) might have a rare exception pop up, but don't hold your breath.
* Be prepared to search extensively: This will require patience and persistence. You may need to search across multiple states or even countries.
* Consider a different make and model: Accepting that a manual transmission Buick Regal from that year is highly improbable might be the most realistic approach. Many other car manufacturers offered manual transmission options during that time.
In short, finding a 1998 Buick Regal with a manual transmission is a significant challenge that likely won't yield results. Adjusting your search parameters (year, make, and model) will greatly increase your chances of success.