There are several courses that you can take online to become an RDA in the state of Texas. If you visit the state board at www.tsbde.state.tx.us/ you can find all the information you need, as well as a list of approved schools. On this list, there are a few "online" courses that you can take that consist of an 8 hour lecture, after which you take an exam. If you pass, you will submit the test scores to the TXSBDE along with a fee and they will determine if you are approved. Once approved, don't forget to activate your registration.
CDA is different though. The CDA is obtained through DANB. This is a nationally recognized exam, and is typically recognize in 28 states. You may have to take an additional Jurisprudence exam in the state you would like to relocate to in order to ensure you understand their state laws.