Both are acceptable, and understandable; however, the second is most likely the best. There are very subtle differences, though. The reason for this is in the tenses.
Sentence B is purely future tense; reaching out, letting you know, and finding are all the same tense. Therefore, one would understand that the three things are related, and are to occur in order: we'll reach out, THEN get back to you with what we find. The implication is that the finding is related to the reaching out.
Sentence A, uses a different tense for the finding. "What we have found" uses the perfect tense (usually used for something finished or completed), in contrast to the future tense of the rest of the sentence. This could be taken two ways: first, as the assumption that something definitely will be found as a result of reaching out; or second, that 'what we've found' is unrelated to the reaching out. The first would be assumed unless the context dictates otherwise.