Secret DNA tests are not the same as anonymous DNA tests. Secret DNA testing make use of found DNA — alternative sources of DNA, sometimes referred to as discreet DNA specimens, that do not require that the person providing the specimen be aware that a DNA test is being conducted. Whereas anonymous DNA testing is an administrative process designed to keep the identity of the test participants secret. It is possible that both may apply to your case — you may with to conduct an anonymous DNA test while keeping the testing process secret from some or all of the participants.
Buccal swabs are the most reliable and least costly way of obtaining DNA evidence. However they require that the test participants be alive and cooperative. You can consider discreetly collecting alternative sources of DNA when buccal swabbing is not practical due to the test subject being deceased or unwilling to participate in the test.
Please review our Sortable & Searchable Table of Alternative DNA Sources which is designed to assist you in selecting the least costly source of DNA that fits your circumstance.