A "legendary source" is a permanent, spell, or card in any zone with the supertype "legendary." Notably, a creature that becomes legendary due to Coronation of the Wilds is a legendary source
Another idea is putting in more dumb value bodies that stack up but do something like give you a land or making defensive creatures that can be sacrificed for a card
in fact, Scurry Oak is probably your best target for any ability that lets you choose which creature gets +1/+1 counters
The Pokmon Company designed this game around trading, so having good cards to trade and knowing how to find out whether a card is rare is vital to not getting ripped off
An infamous example from Shadows Over Innistrad was for Descend Upon The Sinful, which in French was translated to "Fondre sur les pcheurs", which, thanks to the incorrect diacritic on pcheurs, means Descend Upon The Fishermen
Of course, you need additional land cards to make this effect worth it