It's pretty easy to "vaccine shop" (if you care about it) at least in the US with all of the scheduling being done online. Sure, if you go to one of the big vaccination sites, you may not get a choice, but it seems that sites offering J&J have been advertising that and ones that don't indicate seem to be Pfizer & Moderna (or if J&J, give you a choice, at least around Massachusetts).

J&J is also widely used in the US for people who might have issues with scheduling a second shot, for example people who are homeless, or are home-bound.