One of the items I collect from Japan's Sailor Moon Store is the otedama (which looks very similar to Tsum Tsums). From about a year or so ago, they started releasing them in various sets. So far, there are 23 of them including inner senshi, outer senshi, Chibi-Usa, Tuxedo Mask, the cats, Black Lady, and more. These are super adorable! They cost 650 yen (tax not included) each, but most people sell them for between $10 - $14 USD a piece. Since these are only available through the physical stores or via Pretty Guardians Fan Club (Japan only), you'll need to find an online proxy service or buyer to help obtain them.
The next set to be released is the Shitennou (set 5), which I'm very excited about because there aren't a whole lot of merchandise of the four generals. The price for the set is 2,600 yen (tax not included), which comes out to 650 yen a piece. I guess they aren't doing any discounts for getting a "set".
My verdict: While I don't collect Tsum Tsums, I do collect these since they are Sailor Moon (of course). If you are a fan of Sailor Moon and Tsum Tsum, then these are perfect for you! I really enjoy these because they are ridiculously cute, great quality, and doesn't take too much space.