[Spirit's] can also be used as [Spirit is], just like [it's] is a contraction of [it is]. That's not really appliable here either, so I sort of ignored it. 8D
Yeah, Spirits would be right as a plural. It's really up to you if you want to leave it as plural, as the item is stackable, but since the game doesn't actually name anything as multiples, I'd say Spirit as singular would be more correct, considering there's only one spirit in the item image.
I gave it a look-over, and it's actually pretty nice for a fansite. It's no touhou wikipedia, that's for sure. (Doesn't even have close to the amount of information), but it is built nicely. Unfortunately, I can't say there's a lot on that site that I don't already know about, though. ;
well you should give their game collection a look. they have all official games and (i thing) all fangames as downloads for free.