At one's elbow refers to something or someone being close by and easily accessible. There are a variety of synonyms that can be used to convey this same meaning, such as "by one's side," "at hand," "within reach," "nearby," "adjacent," "beside," "close at hand," "in close proximity," "within arm's reach," "within grasp," "within striking distance," and "within sight." Each of these synonyms can help inject a sense of immediacy and accessibility into a piece of writing, whether it's a formal document or something more casual. When choosing which synonym to use, consider the tone and context of the writing, as well as the precise meaning you're trying to convey.