Jenny, I had issue finding daikon too. But was in Asda the other day and the sell them, maybe not all asda's depends where you are, but might be worth a look if you have one near by? They have an exotic veg section
Sorry to burst the bubble but the paste is just not the same. You can not use the ume boshi paste for the plum itself. Not to say that the paste is not good, if you're using it for a yummy butter or salt it's still good but you'll have to shell out for the plums, I found them on iherb.com for $18.27 which is the best price I've found. Keep in mind that you do NOT use a whole plum most of the time. You use small piceses, some times quartered so they last a LOOOONG time. Invest in them, they are worth the price and they last!