ISO9001-2008 requires Companies to have a mechanism to assess their suppliers and continually evaluate the overall products and services they offer you.
It is not necessary to assess every single supplier who you might purchase from. It is more important to control your critical suppliers who do supply products and services that affect your product directly based on specific work they would need to do for you.
For example: if you are sending your product for value added work, such as welding, powder coating or any other process work, it would be a good idea to assess these suppliers to ensure they have mechanisms to adequately deal with your request. If it is products you can buy off the shelf, you might want to assess this type of supplier on a much more low key type review.
Make your system work for you and don’t develop it only to meet ISO9001-2008 standards.