The viral vector vaccines (J&J, AstraZeneca) are much better than nothing, even against Delta. But they're also substantially worse than what you get from the mRNA ones (Pfizer and Moderna), especially the single-shot J&J (study, NYT). People who got J&J could likely benefit from an mRNA shot, and while the CDC has not recommended it some places like SF General Hospital are offering supplemental doses.

One of my housemates had received J&J and decided to get a supplemental mRNA shot. Here's what worked for them:

1. Got to the nearest CVS.
2. Ask what vaccine they have; if they only have J&J, try somewhere else.
3. Say yes when they verify that this is your first shot.
4. CVS asked for their name and birthdate, which they gave, but they weren't asked for ID or insurance. Possibly if you are you would need to decline?

This requires lying to the pharmacy, never something to take lightly, and there could potentially be negative medical/legal/financial consequences, but with the information we currently have it seems like something to consider.