Can I play an attachment on a character from a different sphere, if I pay with the correct sphere resources. Example: I pay with a tactics hero's resources for Horn of Gondor, but instead of playing it on a tactics hero I play it with a spirit,lore,leaderhip hero?

You can play the attachment on every hero or ally, no matter the sphere, but you have to pay it with the correct resources.

Usually if there are some restrictions on characters (hero or ally) on where you can attach the card, they are written on the card itself.

Ex: "Blade of Gondolin" (Core) only allows to be attached to a hero;

But "A Burning Brand" (CatC) restricts to only attach to lore characters (either ally or hero);

Hope this help…

Thanks a lot. I needed the answer a lot. Thanks