Synchro Boost is a Equip Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Synchro Boost is Unlimited.
YGOPRODeck lists Synchro Boost for these formats: Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Synchro Boost appears in Starter Deck: Duelist Toolbox (5DS3-EN028) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Synchro Boost is about $0.04 on Cardmarket (prices change frequently).
