There are some exceptions along fusion summons. Here are two examples of those.
1. Dragon's Mirror
Dragon's mirror is a fusion-summoning spell card like Polymerization. However, players pick fusion materials from the field or the graveyard and put those materials to the banished zone instead of the graveyard. The spell card can be used for only summoning a dragon-type fusion monster.
2. Ritual Beast Ulti-Cannahawk
To summon the monster from the extra deck, players do not have to use summoning card, such as Polymerization due to the effect of the card itself. The fusion monster allow a player to summon itself by picking required fusion materials from the field and put to the banished zone.
You can get 1 from Joey level up reward 4 I believe.
The other copies are bought from the Card Trader.
You can also get other types of Fusion cards from boxes
-Fusion Gate as the field spell!
Fusion Gate field spell rocks in a lot of PVE decks!