I recently used a cheapo set of earbuds to adjust my levels, and the results were good. Unlike headphones, they tend to have pretty good isolation and don't stand out.
I have seen the Shure SE215 earbuds recommended because of their clarity and isolation. Adorama has them on sale for $89. However, I went cheap and ordered a set of Vsonic's from
Lendmeurears.com for something like $30. Not here yet to report on. They were also on sale.
We carry everything else. What more is it to carry a good set of earbuds? Seriously. Try it.