I obviously ordered my GT1 from SimLab. Apart from some shipping issues (made worse for the fact that I am in Australia and the turn around was longer than usual for local customers) they were very good in accommodating my requests. I bought no additional hardware apart from some bolts at the local hardware store and all was sent via SimLab to me. I paid for the additional pieces in the initial order. If you want a GT1 with some extras they should be able to sort that out for you. Some people have reported that their P1 rigs came with additional profile left over from the build but mine did not, apart from what I previously had requested.
Apart from that, if you need profile later on for mounts that you didn't account for in the initial build, there are places on eBay that sell profile and accessories, or there is an 80/20 website that specializes in all things aluminum profile. Online you should be able to source someone that is local that won't cost you too much on postage should you need to add to you chassis later on. For anything you already know you will need, I would just hit SimLab up and ask them to fill in any special requests or needs you may have and give you a price on the regular order plus the extras.