rubber thread as elastic replacement.?

Hi. I had a question, have you seen elastic thread? just a week ago, my aunt showed me a one peice dress which had normal straight lockstitch, but it acted like elastic.The thread on the inside was streachy and was acting like an elastic. Ever since I saw that, I kept googling it to buy a spool , but to no avail.Can you suggest some search keywords?

Here's a good one:

I don't use elastic thread a lot. It can be bit tricky to get it to work well in your machine, but I encourage you to try it! I think you can only use this thread in the bobbin, not the top thread.