I've been looking at penal substitutionary atonement and am finding that although Calvinists and Arminians seem to affirm it, it appears to be Calvinists who affirm it most strongly.
Is this accurate?
The strongest defenders of penal substitution are going to be conservative Calvinists: that is, those who hold to the "five points" of Calvinism and reject modernist approaches to Scripture. Outside this group, many nonetheless hold to penal substitution, but there is more diversity of opinion.
A few examples of conservative Calvinists who hold to penal substitution are R. C. Sproul, Louis Berkhof, Charles Hodge, and Wayne Grudem. Harder to find are those that question the theory; one example of a Calvinist who has been criticized for innovation in this area is John McLeod Campbell, a 19th century Scottish theologian. Even his views, however, are described by some as de-emphasizing the penal aspect of atonement, not jettisoning it.
In the broader evangelical movement, many reject one or more of the "five points" of Calvinism but nonetheless hold to penal substitution. A prominent example would be Calvary Chapel, an association of churches whose statement of faith says that in his death Christ bore God's wrath on our behalf. But there are exceptions here as well; for example, C. S. Lewis was not particularly comfortable with this view.
In more Arminian circles, particularly in Methodism, penal substitution is less widely held. John Miley is a key figure in this respect; he preferred a governmental theory and was highly influential in 19th century Methodism. Still, even here some who associate themselves with "classical" (i.e., non- or pre-Methodist) Arminianism hold to penal substitution.
And finally, among those influenced by Modernism, penal substitution is often rejected. Unsurprisingly it is opposed by the likes of Schleiermacher and Ritschl, but even a "conservative Modernist" like Karl Barth was not fully satisfied with penal substitution, though his views on the subject appear to be a matter of some debate.
In general, you are right: penal substitution is most closely associated with Calvinism and evangelicalism. Beyond these groups it finds some adherents, but not as consistently.
Theory suggests that Calvin's strong support for and writings about penal substitution was tied to his work in the legal system (as a lawyer). The concept is definitely a parallel.
[Christ] made a substitute and a surety in the place of transgressors and even submitted as a criminal, to sustain and suffer all the punishment which would have been inflicted on them. - Institutes 2:16:10
It is my opinion that penal substitution seems more closely tied to Calvinism due to misconceptions on the doctrine of limited atonement. Though limited atonement is a pillar in Calvinism, Arminius believed that the efficacy of Christ's payment was limited to the elect based on the foreknowledge of their faith (Article II - The Five Articles of Remonstrance).
All that aside, there is quite a bit of scholarship that suggests the early church historically and uniformly embraced the theory of ransom atonement well before penal substitutionary atonement.
The oldest theory is the Ransom Theory...It held sway for a thousand years. - The Cross of Christ by Vincent Taylor