This article is from the alt.usage.english FAQ, by Mark Israel firstname.lastname@example.org with numerous contributions by others.
The forms you're likely to encounter, in roughly decreasing
order of formality, are "that kind of thing", "those kinds of
things", "those kind of things", and "that kind of a thing". Sir
Ernest Gowers wrote: "it is as well to humour the purists by
writing "things of that kind"."