Cheryl, I didn't find anything called "Summertime" but there are two Echibeckias with the name "Summerina", an orange and a yellow one.
If you just Google Echibeckia Summerina, you will get several nurseries who are selling it online. I'd caution you that it is quite late in the summer to be starting perennials from small transplants and online sellers will be selling small transplants because big ones cost too much and are too fragile to ship.
Also, it's risky to buy plants online in hot weather. If you aren't home when the package arrives and they sit outside in the heat for too long, you can come home to dead plants. Been there, done that.
So your other option is to contact a local nursery and ask them if they can get plants for you. That way you are not taking the risks, and you might get bigger plants to start with.
To be honest, I'd be looking for a source for seeds at this time of year, and start your own plants that will be ready to plant out next spring.
