By "overo" I assume you mean Frame? Actually in the pinto world (including all breeds) Frame is alot rarer than any of the other patterns, previously Splash was thought to be the rarest form of pinto pattern but since we know understand how it's controlled and expressed we're seeing it alot more!
But yes there are overo Shetlands. Lewella's stallion, Bear, has all 4 pinto patterns (tobiano, sabino, splash and frame) and is homozygous for black, tobiano and splash (if I'm remembering correctly that is).
Country Star Farm (
http://www.geocities.com/overoponies) is one that breeds for overo Shetlands, they also have minis. Lewella can probably tell you which Shetland lines are best known for overo patterns. Interestingly UK Shetlands only have Tobiano and Splash.
Alot of Shetlands are only tobiano by the general (incorrect) standard that tobianos can have the same face white as a "solid" horse - we now know that all facial (& leg) white is due to at least 1 pinto pattern and a horse really isn't "solid" unless it doesn't have
any white at all. So a tobiano Sheltand/Mini that has any face white, will also have at least one overo pattern.