shaymin actually has an abundance of merch over the years, both sky and land formes! it makes it quite hard for collectors haha, there is just so much stuff. often it is japan only, and even more frustratingly, from gashapon, ichiban-kuji, ufo catchers, or some other kind of lottery/blind box type thing. shaymin has also gotten a ton of newer merch as well. in 2016 it was the july mythical pokemon for the 20th anniversary, and 2018 was the pokemon center's 20th anniversary, where it was co-featured along with pikachu! takara tomy continues to put out shaymin merch as well, they make a lot of the figures and plushies (recently they put out a landmin plush in january of this year, its very cute). you have to dig a little deeper to find it, but i assure you there is tons!
if you're interested, there is a wiki dedicated to collecting pokemon merch that compiled a list of both landmin and skymin merch, called pokemon collectors. it is a little outdated but still serves as a decent checklist. sorry for rambling!