Menards is blowing you smoke, Mike! And, they're just too lazy! I've heard of several Lowe's around the country that will special order 4x10 ... it might take a couple of weeks, but they'll get it for you. Most big box stores just don't want to bother with it because it won't be a big-profit item ... that's why I recommend you find an independent lumber yard that is still service-oriented.
Capitol Plywood is the West Coast source for 4x10 & 4x12 plywood (and I think 5x9, which is standard Ping Pong table stock), and I've heard that Roseburg Lumber is also a source (though I've never bought any). I get my 5x10 and 5x12 from Hardwood Specialties here in Redding, CA, but they've been pretty tight-lipped about their source ...

...! There have been several builders in Southern OR and all over NorCal who have driven to Redding to buy the 5x stuff from HS ... but I'm pretty sure a trip out for you from WY is out of the question ...

...!
Good Luck, and as always ...
CHEERS!
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