The phrase "suppose for instance" is correct and usable in written English.
You can use it to demonstrate a hypothetical example you are about to describe. For example, suppose for instance that I decide to take a two-month sabbatical.
Suppose, for instance, the current leak rate turns out to be 4 percent.
Suppose, for instance, employers have monopsony power as buyers of labour: they will curb their demand, to drive wages lower.
Suppose, for instance, you supervise a promising PhD student who could one day become a scientific leader.
Suppose, for instance, that they do win, but by a single point (and also win the Electoral College).
Suppose, for instance, that one has a promissory note that is a promise to pay one $100 in one year's time.
Suppose, for instance, that you think the "Etch A Sketch" comment made by one of Mr. Romney's advisers could harm him.
Supposing, for instance, that the fundamental relations of pitched sounds in the science of harmony and of musical composition were susceptible of such expression and adaptations, the engine might compose elaborate and scientific pieces of music of any degree of complexity or extent.
Suppose that, for instance,.
Liza's anxiety is not about keeping up; it's about acting her supposed age — for instance, she can never quite get the hang of a meme.
Ross supposes, for instance, in accordance with long tradition, that the fact that one has promised to do something is always some reason to do it.
Finally, although an utterance of (5) is likely to express a falsehood — assuming Fred is a rational agent — such an utterance may nevertheless communicate something true (supposing, for instance, that Fred has two distinct 'modes of presentation,' or ways of thinking about, the famous Roman orator).
