You'll want to take a look (I think) at Konami Official KASCs, Ransai ASCs, and Desktop Arcade DASCs. Depending on your time frame, you might also consider a DJ DAO.
Konami officials or
KASCs can really only be bought secondhand (try eBay or the Marketplace here) and tend to be the most expensive. I've seen anywhere from $300-$450 for those, and you'll probably have to wait for quite a while for one to come up for sale outside of Japan.
DASCs can be bought
here - they just brought out their "V4" controller, which runs $200 (or $225 with a super-special paintjob) plus shipping. They also have a smaller controller for $125. DASC has some stock controllers ready to go right now, last I checked, so this would be your fastest option.
Ransais can be bought
here. Basic Ransais run $250; Ransais with lights, fancy paintjobs, button art, etc., run closer to $300. Right now Ransai has a pretty long wait going (I'd estimate
at least 6 months and probably at least 9), since they're bringing out a line of IIDX ASCs and have a huge order queue; however, if you want a Ransai and don't want to wait, you can probably pick one up secondhand for $200-250 shipped.
DJ DAO Pop'n controllers do not actually exist yet. However, many people have had good experiences with the DAO IIDX controllers. These are factory-made rather than made by hand, which may be a good thing or a bad thing depending on your personal preferences. They're supposed to be available for orders in a week or two
here. According to kaizoku who's a semi-official spokesman, "not last price,but we think DJdao POP ASC pirce must be under $200."
If you need non-price information, definitely search around Bemanistyle. I'm a Ransai lover, as you could probably guess. I went secondhand because of the wait time otherwise, and I've been very pleased. I'm currently modifying my controller's artwork and installing lights. Eventually (assuming Random's consent) I plan to put a video of the latter process up on YouTube for people who'd like a step-by-step walkthrough... but that probably won't be in time to be of any use to you personally. Others on this site have had great experiences with controllers of all stripes. I recommend you check the threads for each type:
DJ DAO,
Desktop Arcade. Unfortunately, there are no Ransai or KASC threads that I know of.
Eek.
I'm going to shut up now.
pop'n asc 
Thank you mister! I'm glad you're enjoying it (and progressing, from what I've read in your other posts).
