Thanks folks!

When I search for 火车票代售处 in Google maps, there are several that show up around Chengdu and Google calls them "Train Ticket Office" or "Booking Office for Train Tickets". Looks like there is one in Lijiang as well, so I should be able to get my train tickets before I arrive in Chengdu.

Google's not necessarily comprehensive or up-to-date in China because of the Government's restrictions, so I'd just ask hotel reception for the nearest train ticket booking office.

Note that while train station ticket offices are usually open 24/7, these small offices keep business hours, and may not open on Sundays.