For factory replacement parts, rockauto.com is usually a good place to start.
As far as aftermarket parts, there's always the old standby of JC Whitney. Most aftermarket companies and even sites tend to specialize though, so you may need to look for something specific to the Highlander if you are looking for anything significant in terms of non-factory parts. That said, I doubt that there's much of an aftermarket for Highlanders.
ZV