I am over 65, so I just made an appointment at Walgreens and had the Pfizer booster--that was my original vaccination--and flu shot at the same time. A few hours after I signed up, actually.
I believe that the screening questions would probably allow an under-65 person with colon cancer to get the booster, if it has been six months since their last vaccination, but if in doubt getting some kind of authorization from one's PCP or oncologist would probably do the trick. In the US, anyway.