Ben Cox commented on Direct link to an .xml file

You should be able to find it using our Asset resource:

http://docs.shopify.com/api/asset#index

You can either do this call for all assets:

GET /admin/themes/#{id}/assets.json

Or this to receive a specific asset once you know the key and theme id:

GET /admin/themes/#{id}/assets.json?asset[key]=templates/index.liquid&theme_id=828155753