I’m looking at getting in to Road Cycling in the new year. It’s been a few years since I was last on two wheels (and I had an engine between my legs then

).
The main reasoning for getting a road bike is to use with a Turbo trainer for HIIT work in addition to weightlifting/powerlifting, though it has the upside of allowing me to get out and about and involved in a new hobby too.
I’m trying a friends this weekend to see if I actually get on with it as a form of HIIT first but would be interested to know if there’s a particular time of year that would be best to look for a used road bike? I expect to get more bang for buck buying used.
In addition to the usual places like eBay, Gumtree, preloved I’ve also come across onceridden & bikesoup. Is there anywhere else worth looking? (in addition to any local shops)
I'll come back with a budget when I am certain, from my limited reading an aluminum frame and carbon forks should be on the list and I *think* (based on Wiggle's guide) a 56cm frame should be the sweetspot for me at 5'10 with a 30" leg.