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
12 Black GT500 SVT PP l 2.6 Pulley/Idler l Airaid l o/r X l 305 NT555R's l Recaros l
04 Azure Blue Mach 1 / 03 Sonic Blue 3.8 / 14 Sterling Grey F150 FX4 402A 5.0 / 11 GT500 R.I.P.