Self serve wreckers are a good place to start for common interior trim parts (anything that you would also find on an Executive). Sometimes you can strike it lucky and there will be a Berlina/Calais/Statesman you can salvage some interior parts from. Usually the interiors at wreckers are very dirty/dusty from people leaving doors/windows open, but if you are looking for plastic parts they come up a treat after a good clean. Check for scratches because usually people are pretty ham-fisted and also most of the engine parts end up thrown in there too lol.
Other wreckers where they dismantle the parts and shelve them for you can be better for finding rarer upmarket interior parts, but my personal experience with these wreckers are that they are usually rude, and they ask way too much for the parts. That's just my 2 cents - i don't even bother with these type of places anymore.
Ebay can be a good source for some of the rarer upmarket Berlina/Calais parts (storage compartments, woodgrain trim, etc). Sometimes you can get a bargain, other times it ends up in a bidding war and goes for a ridiculous price. Don't be too bummed if an auction goes sky high, just leave it - there is bound to be another item come up in the future. As always with Ebay don't buy from people with minimal/bad feedback, if the pictures are bad quality and they don't mention the condition assume it's in bad condition, and if it looks too good to be true it probably is.
Most interior parts you can buy brand new from Holden, but be prepared to pay through the nose. A simple center console compartment for my VY Berlina was $100. I think on the VT some of the compartments have a removable clip, so you can get a new replacement clip (for a couple $) rather than having to buy the whole thing.
You eventually work out what parts of the interior are prone to breaking. I myself am very careful with my interior, but still have a small stockpile of spare interior parts that i know will eventually break or wear out!
For the seats, try renting one of those carpet steam cleaners from the supermarket with the upholstery attachment. Mighty mods have a video about those here:
How To Steam Clean Car DIY - YouTube
For the windows, you will find there are little nylon runners that attach to the window regulator arms and slide in window rails. These are very prone to breaking and will cause rattling and grinding noises when you put the windows up and down. You can simply buy new plastic runners from Holden, remove the old runners, clean out the rails and relube them with silicon grease, then install your new runners. Then your windows will operate like new (hopefully). Theres a guide here:
https://forums.justcommodores.com.au/threads/replace-plastic-window-runners.123775
