Hello & Welcome,
Congratulations on your radio purchase.
K40 has a good reputation for quality antennas, including magnetic mount style. I have a link to an Amazon listing for one such antenna that might suit your needs.
Click Here --> http://amzn.to/2dN7DdG
I also have a link to a "search" I did for "magnetic mount cb antenna" on Amazon, that will show you a wider selection.
Click Here for Search Results --> http://amzn.to/2dwMWPN
NOTE: You MAY be able to find similar choice in your local area, depending on what electronics or CB shops you have around.
Please let me know what you find or if you have any more questions.
If this has answered your question(s), please take a moment to rate MY service level (5-star is my goal) so that I get credit for my time helping you with this question.
You should see some "stars" (and SUBMIT button) to do that, but please let me know if you do NOT see them (you may have to scroll UP or DOWN) or can not use them.
Thanks, ***** *****