There are quite a few online sites that sell Makkun CDs and videos. The prices vary widely depending on shipping methods and special discounts, so below is a little guide on where to get the best deals.
Ratings are out of 4 's.

Amazon.com (US)   -    
Offers a few Okui Masami CD albums as special imports. Has quite a few shipping options, including "super saver shipping" (free but slow shipping within the U.S. for orders totaling $25 or more). However, don't think the same online company that gets you US-printed books at bargain prices will do you the same favor for you on imported music. CDs are greatly overpriced (ie, US $53 for "NEEI"). Look elsewhere.

AnimeNation (US)   -    
One of the largest anime/Japanese music stores currently on the web. Unfortunately, it has a very limited selection of Masami Okui CDs and no known VHS/DVDs (as of December 2002, it only had "crossroad" on its inventory, and that wasn't even the latest Makkun release at the time). Very small selection of general J-Pop and J-Rock music, but anime / game music CDs have much better representation.Fairly high prices.

eBay (Worldwide)   -    
Everyone knows about this popular online auction site, but did you know you can also find some rare authentic Japanese CDs, videos, pop idol photo albums, and other music merchandise here, all at (potentially) very low prices? While you're whetting your appetite for that lucky 1-in-1000 find on eBay, however, be sure to keep an eye on those other 999 bootleg CD & DVD sellers... not to mention your fellow frugle (and viciously persistent) eBay bidders.

CDJapan / Neowing (Japan)   -    
Company based in Japan, has the most complete inventory of Okui Masami goods I have seen online. Offers some relatively low prices, even with the more expensive (but speedier and safer) EMS shipping method, and also accepts pre-orders of most releases for those people itching for first-pressing editions. Website layout is clean and ordering is very easy. All in all, a great site.

YesAsia.com (Pan-Pacific)   -    
Offers an impressive selection of Japanese, Korean, and Chinese music. It also has a decent inventory of Makkun stuff, including her more recent albums and a handful of singles. Accepts pre-orders for releases from more popular music artists. Prices are low and free shipping is offered for larger orders (over US $49), and they have branches all over Asia and even in the U.S. for speedy, safe shipping. Be careful, though, of their so-called "overseas versions" of CDs and videos; they're just bootleg CDs with a deceptively innocuous name (unless you live in Hong Kong or Taiwan or something). Overall, however, excellent online store.