You can use some of the proxy services listed in @smpratte 's Japanese Marketplace List. I made a post on the second page of that thread, linking directly to the Pokeémon TCG sections on those sites.
You can search for “ポケモン ムンク展” on those sites to find the various promotional items from this event. Mercari now has a couple of buy-it-now listings for 2,333 JPY (~$21), like item.mercari.com/jp/m54474320050. To purchase from Mercari you’ll want to use a service like vvjapan.com - I’ve used them several times in the past and their service is great; just be warned that they’re based in Japan and it’s a manual service, so you’re not going to get a quick response if they’re fast asleep.
I’m not Japanese, but I can say that they’ve been selling on Mercari for around 4,000 JPY (~$36) - here’s one which is for sale right now. After postage and possible import fees, you’d probably be better off buying one for $50 from eBay.