Several times before I've mentioned dora_ts17, from whom I've probably got a majority of my physical discs at this point. He's got a huge selection, BUT he isn't the only game in town so I thought it would be good to have a place to mention other good people to get H!P stuff from.
I just got some stuff from spiderpage. A smaller selection of H!P specific goods, and unfortunately things aren't categorized so you kind of just have to browse through a few hundred items. At first glance the prices appear to be a bit more than dora's, but some (not all) have free shipping which largely cancels it out. Another big bonus, at least for those of us in North America, is that her items ship not from Japan but California, so the shipping takes days instead of weeks.
Wanted to say that it's rather funny the first seller has a lot of H!P stuff because they also have a good number of Yu-Gi-Oh cards of both languages. I'm an avid card collector to, so if I do want to buy from him then I can get more than just a H!P item haha :)
It doesn't happen automatically, but if you do buy multiple items from dora_ts17, when it's time to pay make sure to do the "Request total from seller" option to save a bit from just having all the shipping costs summed up.
A new seller to point out, mcarmar-jp. This person doesn't sell music, though, but bundles of other old items, largely photos. I've normally not been interested in these when seeing them online. Buying a handful of individual photos for a few dollars plus shipping? But a bundle of 100 of them plus other things at a much lower per-item price is a bit more interesting. Someone could probably make a bit of profit if they wanted to go to the trouble of reselling these things in smaller quantities. I've been paying attention to this seller for a few months--they don't always have an H!P bundle listed, but I haven't seen it go for more than a few days without one.
I've purchased a few lots from mcarmar-jp; I should post some images of some of the things I ended up with on the forums or a blog post.
Berryz (and C-ute) albums don't seem to show up nearly as often as Morning Musume ones. If you do something like search for Berryz at Amazon you can often see them listed by resellers. The prices are usually high compared to the CDs on eBay, but I guess this is a case of supply and demand.
I've just run across eBay seller music_horizon 1. No transactions complete yet, but looks like a large variety of Japanese CDs at decent prices. Most are Buy It Now $5, with shipping being $3 for the first and 2.50 for ones thereafter--or lower shipping if you choose to take the "without jewel case" option. Most of them, at least ones I recognize from H!P, are albums rather than singles.
Only a small fraction is Hello! Project, but it's not like it takes long to skim through. And you'll probably see fewer than I do now since I'm grabbing up some I haven't seen elsewhere, but there are plenty that were already in my collection anyway.
Not a note about a new seller, but an old one. In recent months dora_ts17 seemed to be having fewer of the things that initially got me paying attention to his store, and when I'd check a lot of it would be the same photobooks or expensive DVDs that have failed to sell. HOWEVER, there's now a batch of hundreds of varied items ending in 2-3 days. Full of a ton of stuff I'm interested in, but bankrupting myself getting it all would be a Bad Thing. Singles, albums, single Vs, concerts, DVD magazines, etc. Direct link to his H!P section sorted by auctions ending soonest[/url.