I'm from Australia and thought it would be very hard to find them. Fortunately I stumbled upon a store that not only stocks the plums but also the paste and vinegar. Not to mention many other items recommended in the book. You can find them at: http://www.flannerys.com.au/
Hi, here in my city (Brazil) I found umeboshi that are less salty, the label says ume zuke. Their taste is salty and sour, like the regular umeboshi, but less shocking to the mouth. I take one a day and it´s really helping me with sugar cravings and overeating (don´t imagine why). But I don´t chew the stuff, I let it on my mouth till it disolves, and still I have to think of how beneficial they are to my health, because I really want to spit them every time, the taste really sucks.
I went to my local Asian market(s) for these plums for 5 oz (approx. 12-15) it was $2.99 yes 3 bucks. They also sold them in 10oz for $6.99, this is an everyday food for them, so very cheap. I also got fresh Udon noodles and tofu extremely cheap there too. Whole Foods was just too much for my wallet.
I got mine from Eden Foods but they were extremely pricey. I keep them refrigerated and use them sparingly. That said, I love them and have eaten them before in my childhood. They take some getting used to if you are new to them but keep trying, they are very healthy. They are extremely salty so try eating a tiny bite once in awhile with other bites of food, don't just pop one in your mouth whole. As you get used to them, you will find you crave them. Your taste buds still need transforming, which will happen over time.