To say or write else's, or elses, etc. is improper English. No matter how you look at it.
However it is obviously considered to be accepted. Much like the word ain't. Else's or elses' would be an equivalent 'slang' usage of the word else.
Rather than to say, "That is someone elses", the proper usage of the word would have to be phrased, "That belongs to someone else". No matter how accepted its usage is, no matter how many people know exactly what you mean when you use the slang form elses or else's, it's simply incorrect. This is actually taught in grade school, say about 6th grade level.
Of course I'm thirty now, so it could be being taught at an early grade. You can verify there is no such usage of the word else, other than as it is the word, else. Check with a New Websters Dictionary if you're ever in doubt.
Most dictionary websites have plenty of broadly accepted slang words or slang usage of words. Websters dictionary is online as well. Simply visit that site and type else in the search. If you understand what the word 'else' actually means, you'll find that it is one of those words that is very commonly misused, even without making it plural.