Questions asks it all. I've tried different .edu sites and some are contradicting, saying that they leaflets were dropped too late, some say none were dropped at all, and others say tens of millions were dropped.
Without a doubt, no, there were no leaflets dropped on them to warn them specifically about the atomic bombings. For Hiroshima they would simply not have done such a thing — the entire point of the bomb was to be a massive, secret surprise, and warning would not be compatible with that.
After Hiroshima they did draw up a leaflet to drop as part of a psychological warfare campaign. They did not get it ready to use, however, until after the Nagasaki attack.
I have written this up extensively here. I consider this report from 1946 on the subject to be the most reliable thing out there on the topic.
All of the sites that assert that there were specific atomic bomb warning leaflets dropped appear to be based on a basic misconception about the difference between them writing up a leaflet after Hiroshima (a lot of them have bad dates on them, e.g. "circa August 6th", which imply they dropped them on or before Hiroshima, but the contents of the leaflets refer to things that happened after the Hiroshima attack) and actually dropping them. The 1946 report clarifies that while they did prepare a leaflet, they did not end up dropping it until after Nagasaki for logistical reasons.
There were separate, "Lemay leaflets" that warned about firebombing in general. They had names of cities on them and said, more or less, we're going to bomb these cities sometime soon, so abandon your posts, etc. It is often asserted that Hiroshima and Nagasaki were included on these lists, though I have not seen a leaflet that anyone has been able to say, "this one contains their names." The most common ones scanned on the Internet do not. That doesn't mean their names were not at some point included on such lists. Personally I don't really think this counts as any real "warning" even if their names were on a list — being on a long list of possible conventional bombing targets is not the same thing as being warned about being atomic bombed. You can hear a conventional raid coming — hundreds of B-29 make a lot of noise. An atomic bombing raid only had three planes and was as such more or less invisible.
I consider the "we warned them before Hiroshima/Nagasaki" one of the most pervasive and problematic myths about the atomic bombings around today, because there has been a long game of "telephone" of people repeating the same bad information about it.