I cant think of anything online that would do that, but GNS3 does have the ability to connect to a virtual machine or you can also use linux based qemu images to run many virtual machines within the GNS3 lab. GNS3 can also connect to a virtual cloud which is routed to your machines actual internet connection. The main issue is, if you have no Cisco knowledge it'll take a fair bit of getting to grips with.
The only other thing I could think that might do it would be to use VMWare and create a virtual network with various clients and some linux based router OS that supports IPv6. I'm not 100% sure how well that would work as I've never really tried it.