Try to index the datasource itself, then you can add a filter subquery fq
to the main query so you can restrict your search as you want.
Say for example that documents datasource are indexed in a field called 'src' and you want to query only documents from 'val_1' and documents from 'val_2' datasources, then appending a filter subquery should do the trick e.g. :
.../solr/yourCollection/yourSearchHandler?q=*:*&fq=(src:val_1+OR+src:val_2)