User blog comment:PegasusMan/Another GOOD Way to get SP Fast for Units - NO BOOSTS/@comment-24026550-20140714065237/@comment-24453214-20140714131504

Also, once you open up Bigfoot Country (level 46), you can raid the spiderwasps there. It's less SP per battle, but you can clear battles very quickly, and you don't have to wait for respawn.