Here's a few sites that carry the spacer for the 98 v6. The only brand I know of is Airaid. You might be able to find them cheaper on ebay, so I'd look around. As far as effectiveness, I don't think their worth the money on a basically stock engine. I feel a throttle body spacer is like a new throttle body, it isn't really necessary until you're pushing more air into the intake.
Another thing to keep in mind is fitment issues. When I helped my buddy put his American Muscle CAI on is GT, it wouldn't fit with the 1" t/b spacer on. We had to take it off for the CAI to fit. I see you have an AM CAI, that's why I mention the fitment issues. I personally think the CAI is more worthwhile than the t/b spacer. Not sure if you'd have the same clearance issues on the 3.8 engine, but you never know.
Airaid - Product Search
AIRAID PowerAid Throttle Body Spacer, 94-98 3.8L V6 Mustang
94-98 3.8L Mustang Throttle Body Spacer - Airaid
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