There is no Lance-specific parts dealer (that I know of), and most parts are very generic anyway.
I have been lucky in the past on ebay and amazon, among others. There are also a number of specialized rv parts dealers. An internet search should bring up a lot of information.
Often the problem is knowing the exact name or part number of what you are looking for. When I am not sure, I try out image searches for terms that I think might describe what I am looking for. Usually clears up the right name/ID fairly quickly. Then I search for that part.
Some things, like old Atwood jacks and controllers, are no longer available new.
2014 Ford F350 XLT 6.2 SCLB + 2017 Northstar Arrow