Hey Mister Strom Sir, my Strom is safely at home awaiting my return, but here's my recent experience buying for the KLR I'm on:
I found a front cheaply and very easily in Ica, just north of Nazca. Everyone in town knows where the moto section of town is, everyone is very nice and helpful, and I paid only US$40 for a Brazilian MT 40. Took a total of a half-hour, no more, detouring from the highway through town.
Then I bought a rear in Cuzco, where I am at this instant: the moto section, which includes Honda and Suzuki parts places, is off Avenida de la Cultura near Estado Universitario. Easily found, less than a mile from the old center. I actually bought from a rental place, Perumototours, Saphi 578, purely because they were so helpful with all my questions, and they mounted it for free (but marked it up about US$5). Also, they're located about a hundred feet from the place I'm staying (Hostal Saphi--pricey at 65 soles but it was the first place I found with parking in the lobby).
I've actually been surprised by the selection available here--better than Central America for sure, and more easily accessed. The practice seems to be that if you buy something from a shop they install it for free, as ridiculous as this seems. For example, I went to buy some oil today and the counter person came out and did my oil change for the price of the (Castrol moto specific 20-50) oil.
Out of curiosity: did your front TKC wear in a very bizarre alternating pattern of sloped knobs? I've had two do that now, and I don't seem to get around to rotating them on the wheel before I'm about to hit cord on the (alternate) worn sets of knobs.
I'm off to Machu Picchu tomorrow, but it sounds like maybe you're gaining on me at last. I'm also headed for La Paz, but thinking of at least passing through Arequipa first. The Gone Penguin folks are thinking Salar for Christmas, and I have my doubts but am thinking of giving that a try.
Stay in touch!