I can find references to each in many online dictionaries, but few will reference both of them. So the question remains:
What is with the difference in spelling between “Locator” and “Locater”? Is one proper and the other a popular misspelling, or is there a nuanced difference between the two words? (Or, even more interestingly, both)
The rules outlined in answers to the earlier question What’s the rule for adding “-er” vs. “-or” when forming an agent noun from a verb? are sufficiently opaque or exception-riddled that I think anyone who wants to follow majority preference in the spelling of an -or/-er word should probably not rely on a generalized rule for guidance. In the case of locator/locater—or any other specific -or/-er noun pair—you can use Ngram search results to get a general idea of which spelling is more widespread in published writing in the Google Books database. Here is the Ngram chart for locator (blue line) versus locater (red line) for the period 1800–2005:
The difference in usage rates looks somewhat more dramatic if we focus on the shorter period 1920–2000:
Merriam-Webster’s Eleventh Collegiate Dictionary (2003) and The American Heritage Dictionary of the English Language (2011) corroborate the seeming preference for locator over locater. The Eleventh Collegiate, which always lists what it considers the more common variant first, has this:
locator also locater n (1784) …
For its part, American Heritage has a full entry for locator but limits locater to a mention at the end of its entry for locate:
locator n. One that locates, as a person who fixes the boundaries of mining claims.
This treatment suggests that some specific uses of locator may require that spelling, while others may be acceptable with either variant spelling.