score:3
I hired gear at a place called Yamagata Swallow, which was in Yamagata City.
It was very close to Yamagata train station, and they had a choice of at least two pairs of 30.5 cm boots. I initially tried emailing them, but didn't get a reply, so a Japanese-speaking friend of mine rang them and arranged the rental. Picking up and dropping the gear off was easy enough.
I had a good time at Gassan, but a write-up of that belongs in another question.
Upvote:5
You could go to Yudonosan. It's just a bit further but distance wise the closest to Gassan with a ski rental service. The tricky bit here is that there is no train access to this place. you need to have a car or take a taxi (17km from Gassan train station). Since the season there ended in March however, I do not know if they are still open when you go there. You might want to call. Since the place is also offering summer activities, the shop might be open for other stuff and still give you a pair of skis.
Also, while there might be a lot of snow there, not everybody seems to think that it's a good idea to go skiing in summer. Every day after it opens, the snow seems to become more and more like a sorbet. I have been to a place like this in Europe (Passo Stelvio) where you can go skiing even in August (did not ski myself, rather passed through, the mountain pass road is right at the ski lift) and I cannot say that it looked like fun just because of the wet conditions:
On top of that, to enjoy Gassan fully, there seems to be a lot of hiking involved.