I bought this tamarind paste online at amazon.co.jp. I tried to find it again but everything was written in Japanese and I just can't find a similar one but I found that looks as good. A 227g bottle is sold for ￥605. Check it out here.
But these are not your only option!
Aside from readily available tamarind pastes, you may go to your favorite vendor at your local market; buy fresh tamarind; go home; simmer tamarind to soften; scoop out the fruit inside the pod and then mash the tamarind and you have your tamarind paste! Just take note that homemade tamarind pastes tend to taste paler than readily available ones so you should consider using more of this in cooking.