There are plenty of synonyms for the phrase "is a source of strength." Some possible alternatives could be "provides support," "offers encouragement," "gives motivation," "supplies energy," "affords confidence", "imbues resilience," "empowers," and "nurtures." Each of these alternative expressions conveys a slightly different nuance, but all share the idea of something that provides positive reinforcement or fortification. Whether it be the support of a close friend or family member, the motivational boost of a self-help book, or the inner strength that comes from one's own convictions and values, finding these sources of strength can be a crucial aspect of overcoming challenges in life.