Dear you need to work as following:

1- Ask every head nurses to complete the related-questionnaire.

2- Ask patients to complete the second questionnaire (nursing services ). 

3- Work on sorting patient answers to groups, each group related to one of the head nurses in the first point above. 

4- Make aggregation to patient answers. 

5- Analysis your data.

best wishes 

 Dear Dr Amer,

Thanks for your answer 

So, convenient sampling method is better to be used than probability sampling method in this case

Yes my dear Alloubani 

Thanks a lot

What is the best method to use for a quantitative studies (a convenient sampling method or probability sampling method)?

Which method to use is also depending on how large the population & your research objective.  If your population is small, you might want to consider convenient sampling as every sample / respondent is precious.  If your population is large, you might want to adopt probability sampling method which facilitates random selection for you to generalize your sampled research findings to the entire population.  Do take note some universities or journals require research based on generalizability.

If your research objective is only confined to a hospital which has smaller number of nurses & patients, you might want to explore convenient sampling.  If your objective is to include many hospitals in an area / country, you might want to use probability sampling method.  Wishing you all the best in your research.

Probability sampling is better than convenience sampling (or any other non-probability sampling). Use probability sampling when there is a need for sampling and whenever possible in order to have valid inferences. 

Probability sampling is always better than non-probability sampling in quantitative study but in some cases you have to choose snowball technique or convenience sampling. It depends on your objectives and capability too.