I've had the Obubu sampler, it is pretty good, but I got it when it was on sale ($15 including shipping). At $25 it's a bit pricey - you do get 100g of tea, but, included in that 100g are 4 kinds of houjicha, a bag of genmai (no leaves), and a bag of pickled sakura leaves- meaning 30g of the sampler isn't really expensive leaf at all.
I agree; Den's sampler is great - also, when you order, you can request other samples that you'd like to try. In fact, I was once trying to decide whether to go with a large amount of the mid or high level sencha (I hadn't tried either one), and simply asked if they could send me a sample of each, which they gladly did (free), and then I was able to decide which I wanted to order.
But yeah, other vendors should start, too!