Tpl for page having url like product/checkout/%/% I have created a page with hook menu with url product/checkout/%/% where % will be userid and product id.
How can I add a page tpl for this url ?
In your page callback, you would have to set '#theme' on a renderable array.

You can use an existing theme function, or create your own using hook_theme.

If creating your own and you prefer to use a tpl, you can set the template option in hook_theme and bypass having to create a theme callback function.