You could look on well-known online movie libraries such as Hulu or HBO Max. Also, some international movie streaming services might have it. Just make sure to have a valid subscription and a stable internet connection.
One possible plot could be that one of Dexter's female friends or a female character in the Dexter Laboratory world discovers she is pregnant. She might then turn to Dexter for help, thinking his scientific genius could assist in some way, like creating a device to monitor the baby's health in a unique way.
You might find what you're looking for on websites dedicated to foreign-language shows, such as Viki. It offers a good selection of Spanish novelas along with English subs. Just make sure to check the licensing and availability in your region.
The Grim Reaper from 'The Grim Adventures of Billy & Mandy' could be an interesting addition. He could bring some dark humor and a different kind of magic to Dexter's world of science.
One idea could be Dexter crossing over with another smart character like Jimmy Neutron. They could compete in a science fair, leading to all kinds of wacky inventions and mishaps.
As I mentioned before, Dee Dee could be a likely candidate. She's always getting into Dexter's business, and a pregnancy could add an interesting layer to her character. It could show a more serious and responsible side of her as she prepares to be a mother.