I agree with making your own...even though you stated you didn't want to.
Throttle return springs are pretty small in real life, and a lot of 1//25 scale replicated versions look like large garage door lift springs. Also it is more comon that the springs have a straight section of each end, and getting that from a pre-made spring is difficult.
Here is a larger diameter spring, but it's probably only 4" or so long (or 4mm in 1/25):
Here is one on a 1/24 scale model:
It's a cool detail to add, but I would make sure it doesn't detract from your model by being too large or looks like a spring glued to the side of a carburetor.
I make mine with fine guage wire. Just wrap tightly around a small pin, slide off, and bend the ends to replicate your paritcular use. Also, making a place for the spring to attach looks better than a spring just glued to the manifold.