search on comma seperated field

 Hello all,
                I have to apply search on comma separated values. I have a table of model in which there is a field
named available for which is of nvarchar type and stores the ids which
are separated by a comma. Now i am working on search module by which a
user can search for a model based on the categories. All the categories
are saved in category table and the main categories have parentid as 0.
 
for eg. in database a model is availablefor '3,6,7,8'
and the user wants to search for a model  who is available for '3,7' . How can i do that?   
...