Both trees, as you may know, have big "issues" when used as bonsai. Birch (almost any species) tend to drop branches indiscriminantly after you work years to build them. It's unstable as bonsai material. Still, some people put up with it.
Sycamore is a huge tree with dinner plate sized leaves--which can reduce with some effort, but they're never really tiny. Additionally, the trees don't ramify branching very well and almost always look funny as bonsai.
Brent at Evergreengardenworks.com has a couple of birch species, but read the fine print on them. "Suitable for large bonsai..."
He doesn't sell sycamore and I've not seen that species sold in any bonsai nursery. Best bet for that one is a local nursery...