I think that's wall paper paste, also used for, um, lots of stuff. Try the hardware store and look at the labels on the liquid wall paper pastes.
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Maybe on Ace Hardwares?
Good choice. But stay off the roof...
You can purchase it here.
Methyl cellulose comes as a soluble white powder with instructions for mixing with water. Refrigerated mix lasts (almost?) forever, and is an excellent gentle adhesive. If you want to remove it plain water (on a tissue for control) will do it. I would recommend a craft store (like Michaels or JoAnn's) to get a pure form. By the way, I think glue violates the principles of origami, which depends on cutting and folding to hold its shape. But if you aren't entering a competition, anything goes.
A craft or art supply store will have it, especially if they carry any significant stock of items for the next two things.
A scrapbooking store would probably have it.
A book-binding supplier will DEFINATLY HAVE IT.
Read labels on the glues at these places.
Wallpaper paste will work, but it may have additives that might affect the paper you're using. You want it pure, in powder form.