For me, it's usually where ever that I can get them. Amazon, e-Trailer, eBay... depends on what it is and if I can even buy it new... Just got my new slide motor from e-Trailer, factory keys (already cut) from eBay, and a lot of smaller components from Amazon.
2008 Fleetwood Discovery 40X
2015 Heartland Silverado 38QBS (BBQ'd due to faulty USB battery pack)
2001 Fleetwood Terry 25W (Sold)