The way I would do this is create custom meta box for the products. One for each external link you want. The meta fields could just be an input field where you enter the external URL of the supplier.
Then in your frontend just hook into the area where you want these buttons to display and go from there.
Here is a reference on how to create the meta boxes
http://codex.wordpress.org/Function_Reference/add_meta_box
And the hooks you're looking for assuming you want these buttons to show on the single product's page is:
'woocommerce_before_add_to_cart_button'
'woocommerce_after_add_to_cart_button'