Where can I buy Konapun?
It would appear that Bandai made Konapun from 2006 to 2009 so it's not as readily available as toys currently being manufactured. But here are a few places you can find it today. Be prepared to spend at least $50 - $60 USD on a kit and $10 and up on refills.
Rakuten is a behemoth of a website which acts as an umbrella for gajilions of Japanese businesses. Many businesses will ship internationally. And if not, Rakuten recommends a "forwarding service." See below for info on forwarding service.
The Japanese Amazon site has sellers selling Konapun. If you are not in Japan / don't speak Japanese, there are articles out there you can search for which give tips on how to order from Amazon.co.jp.
Search Elsewhere on the Interwebs
Search in Google for Konapun or the Japanese word こなぷん and see what you can find. If you don't speak Japanese and you use a browser with built-in translating like Google Chrome, it's very helpful.
If you see any other places that have Konapun for sale and in stock, let me know!
If you find something you want to buy at a Japanese online site and they don't ship to the U.S. or wherever you are located, you can use a "forwarding service." You just set up and account with that service, have the item shipped to them, and then for a fee they ship it to you.