It appears that Nintendo had these gauges manufactured specifically for their use in this product, and they don't seem to be exactly advertising who that supplier may be in order to prevent competitive products from arising. This is common in the manufacturing industry.
As an alternative, I suggest you look towards more generic offerings available from many manufacturers such as the products of Shenzhen Yi Guan Xing Technology Development Company which can be seen on alibaba.com {http://www.alibaba.com/product-gs/1320541037/Load_cell_strain_gauge.html}
These types of manufacturers can supply an equivalent product for a very low price [~10$ USD per piece in small quantities] and they offer a range of products to suit your needs.
I have found alibaba.com and similar sites to be an indispensable resource when looking into small scale production of electronic products.
As for your second question about the "copycat" products out there.... It's often the case that other companies may reverse engineer a consumer product and produce their own variant of it, as it is not generally difficult to do with an item like this. On the other hand, it's not as if what effectively amounts to a modified digital scale is the most revolutionary technical innovation either. I am unaware as to whether Nintendo owns patents pertaining to this products design, or whether those patents are general enough to legally prevent another company from being able to produce a sufficiently "dissimilar" item. There is of course also the difficulty in prosecuting cases like this where the items are likely all being manufactured by some third world country's no-name factory, and simply they are being re-marketed under the various names of EASports, GameOn, etc.....
Good luck my friend!