Hi and welcome - a good choice of car
It can be a bit quiet on here, but Camaro owners (pre-4th gen anyway) are getting fewer in the UK as more and more get scrapped. Thankfully those of us that are left are pretty knowledgeable, and you'll always get some replies to your questions.
Rockauto is good for smaller parts, just be aware you may get some unexpected bills from the UK courier when it arrives. Try and select the options that show you paying taxes upfront and avoid the Economy delivery method that ends up with Parcel Force.
There's some other good suppliers - check NPD, Classic Industries, Summit Racing and JEGS to name a few. For bigger orders or bulky items, look at STS Imports who can save you a fortune in shipping costs - provided you don't mind waiting a few months for sea delivery.
For door rubbers, the latest I've heard is to try and look for ones that have metal inserts in the ends, to help with the fit.
For plug wires I've always bought sets that you have to cut to fit and fit the ends on yourself. I guess it depends on how handy you are and whether you are replacing standard ones, or looking for a more custom look, i.e. specific colour or different routing to standard. Do you have headers or stock exhaust manifolds? Any after market plug wire holders?
For spark plugs I would definitely look at RockAuto - looking for the warehouse clearance deals they do. I've seen plugs selling for pennies in the past. - Don't forget to look for a recent RockAuto discount code - there is always a code around that gives you 5% off.
Any other questions - just ask.
BTW - photos are requried