They're rather hard to find these days, but if you can lay your hands on one the Hoojum Cubit3. Theres also a fair few, rather generic looking, mini-itx cases from companies like Morex and Travla, but you're going to have to find them yourself due to competitor linking rules.
An interesting point is that mini-itx is compatible with ATX, so you could easily fit a mini-itx board in any ATX or mATX case. That would give you a lot more choices, but they would be nowhere near as compact