I found that the harder i worked, the more luck i seemed to have. Is found = is always found this animal is found in australia. A is not correct but was found would work with today etc.
Present perfect (i have found) refers to actions beginning. Coal is found in the northern part of india. And since this sentiment is usually one of those lifestyle maxims, i think we would usually use the first one.
Simple past (i found) refers to actions beginning and ending in the past, without effects in present time.