Ribosomal DNA references sequences
For birds you can start by searching 18s in http://birdgenenames.org/cgnc/

But generally NCBI Nucleotide database, should provide you with all information and accession numbers. See this broad search: http://www.ncbi.nlm.nih.gov/nuccore/?term=18S+ribosomal+RNA+gene

Adding species specification and other parameters, you might find what you are looking for. For example, see chicken 18S ribosomal RNA gene