There is not really any 'set' source for these things. There are a few smaller open-source developers for shopping carts and such - what you really need to do is build your own skills in that department.
Anything that is free or low cost and easy to install is not going to be the best available, be bug-free or have great support.
Best bet is to list out exactly what systems you are looking for and find a great resource for that one thing. You will have a much better chance of finding these individually.