Pretty well done, Petite Fleur! The correct preposition is indeed “in” (it's “to find something in someone”), not “to”. As far as the past simple versus the present perfect, we might well use the past simple in American English, but the present perfect, which would probably be used in British English, is most likely better, unless you’re thinking of a specific time in the past. To me, there is a strong connection with the present (I’d like to thank…). Note that the infinitive is “to thank” or “to give thanks” (not “to thanks”). Note that “to thank” takes a direct object, with no preposition (there is no “a personal” in English :). If we use “to give thanks”, then it takes the indirect object, which we may need to introduce with “to”: - I'd like to thank my coworkers and friends, in whom (have) I found a never-ending source of inspiration. - I'd like to give thanks to my coworkers and friends, in whom (have) I found a never-ending source of inspiration.

Hi, Petite Fleur, I'm a native speaker of American English. "Never-ending" has a bit of a negative connotation. Usually it's used to describe something you wish would end. "It's a never-ending battle to convince Americans to wear a mask." So I would use "endless" instead. You could say, "I'd like to thank my friends and coworkers, who have been a source of endless inspiration/a constant source of inspiration." You can add "to me" at the end, if you like. (I'd tend to put "friends" before "coworkers," since a true friend is usually valued more than a coworker.) Hope that helps!

...

I'd like to thank my coworkers and friends, to whom I have found a never-ending source of inspiration.