CD Japan is DCTJoy's recommended store for all your Dreams Come True needs!
CD Japan is based in Japan, so not only do they have the largest selection available, they can also do special orders for items they don't currently have in stock. In addition to all the latest releases, they also get many copies of limited edition and first pressing items.
Your items are charged at that day's Yen rate, so there's no middle man mark up. The shipping is a bit high, but the speed in which you get your package and the lack of middle man mark up make up for it, especially if you order numerous items. Your package arrives 3 to 4 days after it's shipped, and that includes the time it takes to get through customs (at least in the US).
I've used CD Japan for many years and I can't recommend it highly enough!
Yes, the tried and true Amazon.Com has Dreams Come True CD & MP3s available. Some of the album prices may seem high, but since they're directly connected to the Japanese version of Amazon, they can almost certainly get whatever they're advertising. Just check the number of reviews for the albums.
Their first nine albums are also available for MP3 download.
Another popular place that US JPop fans get their CDs. The US based YesAsia has a decent selection and also gets the cheaper overseas (Hong Kong) versions in addition to the regular Japanese releases. An all around solid JPop store.
eBay can be a great place to find out-of-print, hard to find and unique DCT items, especially older singles. You can also find the regular albums & new releases there for very cheap prices from time to time. It's always a good idea to do a daily DCT search at eBay.