I noticed how hard it is to find IIDX locations online (besides an outdated VJArmy thread) so I'm going to try to compile a list of IIDX locations in the U.S. Not Beatmania because there are too many of those and they are not as popular. Yes, this is inspired by a similar thread in the Pop'n forum. Yes, I'm doing this mostly out of boredom.
I'm building this list by searching DDRFreak so any corrections/additions are welcome. A number of these machines have probably been upgraded.
Arizona Golfland Sunsplash, Mesa - "a very timing-issue-riddled" IIDX Troopers
Gameworks, Tempe - IIDX Gold
California Super Arcade, Walnut - IIDX Happy Sky
Sunnyvale Golfland, Sunnyvale - IIDX Lincle
Milpitas Golfland, Roseville - IIDX Lincle
bemaniland Ranch, Anaheim - IIDX Lincle
Florida Beach Gameland, Clearwater Beach - IIDX Happy Sky
Hawaii Hawaiian Brian's Billiards, Honolulu - IIDX DJ Troopers
Indiana Great X Scape, Bloomington - IIDX Gold
Nebraska Family Fun Center, Omaha - IIDX Distorted
Nevada Atlantis Fun Center, Reno - IIDX Lincle
Peppermill Arcade Extreme, Reno - IIDX Red
Gemini Arcade Palace, Las Vegas - IIDX Lincle
New Jersey 8 on the break, Dunellen - IIDX Lincle
New York Peter Pan Games, Bayside - IIDX Lincle
Ohio Bowling Green State University Game Room, Bowling Green, OH - IIDX Empress Texas Planet Zero Anime Center, Houston - IIDX SIRIUS
British Columbia E-Spot, Vancouver - IIDX Sirius
IIDX Arcade Machine Locations 
