I am afraid there is no original source. Wikipedia has talk pages where sourcing is discussed, and its editors did extensive searches on this one and its variants. It is listed under the heading Unsourced and dubious/overly modern sources, and the "original" appears to have been made up by Ram Dass around 1970. Dass (born Richard Alpert) is an American academic turned spiritual teacher after an epiphany in 1967, known for associations with Timothy Leary and the Hindu guru Neem Karoli Baba. Dass is also the author of another "Einstein" quote dating to about the same time, "I didn't arrive at my understanding of the fundamental laws of the universe through my rational mind". No earlier occurrences of the "quote" have been found so far, Einstein passed away in 1955. Here is the surmise:

"Searching for "Einstein" and "level of thinking" rather than "same level of thinking" turns up a much earlier example from The Journal of Transpersonal Psychology, Volumes 1-4, which is dated 1969 by google books though these snippets show it contains pieces from 1969 and 1970. The quote, on p. 124, is "The world that we have made as a result of the level of thinking we have done thus far creates problems that we cannot solve at the same level as the level we created them at." It's prefaced by "Einstein said an interesting thing", and the same phrase and quote appears in a 1974 book by Ram Dass (who needs his own wikiquote page!), The Only Dance There Is... so presumably the one in The Journal of Transpersonal Psychology is the same piece by Ram Dass.

Also, the next two earliest versions I find on google books (searching for "Einstein" and "level of thinking" on an advanced search with date range 1900-1979) use wording nearly identical to the version given by Ram Dass... The supposed Einstein quote from Skeptic reads "the world we have made as a result of the level of thinking we have done thus far creates problems that we cannot solve at the level of thinking at which we created them", and the one from New Age reads "the world that we have made, as a result of the level of thinking we have done thus far, creates problems that we cannot solve at the same level we created them", both identical to Ram Dass' quote in the first part and very close in the second part.

Since Ram Dass generally spoke extemporaneously without using notes, and The Only Dance There Is says the section with the Einstein quote is from talks he gave to "spiritual seekers" at "the Menninger Foundation in 1970", it's likely he would have been paraphrasing a bit even if his source was some other unknown one that also misquoted Einstein (as opposed to him being the original source of the misquote as I am speculating). So if the only other sources found in the 1970s repeat Ram Dass' exact wording, it seems pretty likely that Ram Dass (or some secondary source that got the quote from Ram Dass) was the source for them. And the later variants that emerged in the 1980s and 1990s might well be all from authors that got their quotes in a chain of influences that goes back to Ram Dass' version."