In Mexico, singles were also released. To my knowledge, they were not released with picture sleeves in the 60s.
Since a few years, Mexican singles appeared on eBay with picture sleeves (mostly from the sellers barbacoa, pokajurecords, fernandnavaheredi0, djcoy74...). Most of them are bootleg sleeves sold with original vinyls. Most of the time the quality of the paper and the print quality speak for themselves once you have a copy in your hand but it's hard to tell from the auction site. It would be easier to avoid them if these sellers would only sell bootlegs but it's not always the case.
Here's a sample of bootleg sleeves I spotted on eBay.