The science for how the mRNA development differs from the J&J vaccine:
https://www.nebraskamed.com/COVID/you-asked-we-answered-do-the-covid-19-vaccines-contain-aborted-fetal-cells
https://www.nationalgeographic.com/science/article/here-are-the-facts-about-fetal-cell-lines-and-covid-19-vaccines
Snopes on the veracity of the claim
https://www.snopes.com/fact-check/covid-vaccine-aborted-fetal-cells/
The debate on the tradeoffs
https://www.usccb.org/news/2021/us-bishop-chairmen-doctrine-and-pro-life-address-use-johnson-johnson-covid-19-vaccine
https://www.kark.com/news/health/coronavirus/arkansas-hospital-lets-staff-seek-vaccine-exemption-as-long-as-they-dont-use-other-meds-tested-on-fetal-cell-lines/ (but see next blog article on why that's not quite right)
https://stream.org/yes-you-can-morally-object-to-a-covid-vax-and-still-take-aspirin/ (doesn't say which of the four on the list were tested during their development)
https://www.patheos.com/blogs/throughcatholiclenses/2021/01/if-any-drug-tested-on-hek-293-is-immoral-goodbye-modern-medicine/
https://churchlifejournal.nd.edu/articles/you-can-have-your-tylenol-too-a-note-on-the-objections-of-some-catholics-to-using-covid-19-vaccines/ (counter argument that post-development testing really isn't that different)
https://churchlifejournal.nd.edu/articles/does-the-origin-of-the-cell-lines-used-to-test-covid19-vaccines-make-any-difference/