> He flipped a guitar upside down because he could source one more easily than a lefty model.

I've read that he claimed he played a right-handed guitar upside down because his father was superstitious and didn't like him doing things left-handed, so he'd play a right-handed guitar upside down most of the time and flip it over when he needed to play in front of his father. (I'm not sure why he didn't play a lefty guitar upside if that was the case, but I could imagine that the availability might be relevant like you mentioned, or maybe his father was familiar enough with guitars to be able to recognize a left-handed one and figure out what was going on, or maybe because he was better left-handed he could play it upside-down well enough but due to not being right-handed he would have found it more difficult to play it in the non-standard way).