A cobble isn't just any old round bit of rock, you know.
No, to be an official cobble it has to be 64 - 256 millimetres across. That's what the geologists say.
Any bigger than that and it's a boulder; any smaller and it's a pebble.
If you can't spot any of that sort of cobble, then perhaps you might find a cobbler, which is either someone who makes or mends shoes, a fruity alcoholic drink, or a hot fruit pie covered in a crust of what I'd call scone mix but which I believe in the USA would be called biscuit.
If you're in Australia you might find Bidens pilosa, a weed known there as cobbler's pegs:
And if none of those things are to be found then I'm sure there'll be something near you that's been cobbled together, or assembled in an insecure way.
Something like this, perhaps:
Or even a blog post...
A load of old cobblers is a lot of nonsense, which will be the easiest thing in the world to spot. My Collins dictionary says that expression is hardly rude at all, but I couldn't recommend it for formal use.
Word To Use Today: cobble. The word meaning stone is probably something to do with the Icelandic kobbi, which means seal (I assume that's the animal sort of a seal). No one knows where the shoe-making word comes from, but the drink may be a shortened form of cobbler's punch, and the word meaning nonsense is rhyming slang, and is a shortened form of cobblers' awls.