My husband and I are both certified teachers in the US but w...

No you don’t but it can make your resume shine better and TEFL is only about $150

You don’t have to have one, but many countries require one for work visas. China requires one, but most employers outside of the biggest cities will fake them.
Korea and Japan want them, unless you have related education and experience, like degrees in education or experience teaching in highschools and universities.
TESOL/TEFL certificates don’t usually have any serial or ID numbers and it’s nearly impossible to verify authenticity. Employers in developing countries will usually fake a certificate to get you the visa, assuming you don’t give them one.
They’re cheap and easy enough to get, so it’s always better to have one. European employers, however, are often picky about which company you get it from. If you want to work in Western Europe, avoid any of the online TEFL programs, Global TESOL, and the 20-60 hr courses.

From my experience teaching overseas in several countries is that the CELTA certificate will get you into many good schools. Also be aware that many international schools require experience or knowledge of the IB programme. Unless you want to teacher in a college. It is worth the money to pay for a certified TESL program. They normally cost between $1300-$2000. This course can be taken at your local college or language schools in your area.

I am also a certified and experienced teacher from the US and I do not have any TEFL or TESOL certificate! I am currently teaching in Istanbul, Turkey and have had many job offers in other countries. The main things that a GOOD school is looking for is that you are a qualified teacher (with an actual teaching degree) with teaching experience and good references, and that you are a native speaker of English.