I think Danny is right. You are best off dealing with a local sailmaker you can easily talk to and maybe even meet face-to-face. It's also important that they are familiar with asymetric kites for cats.

Most sailmakers will need the dimensions of your boat to pinpoint the halyard, pole-end and sheeting point. From that they will be able to design a spinnaker. Unless they've made one for a Mosquito before.
Make sure the sailmaker reads the Mosquito rules. There are some maximum dimensions.

If you just want to go ahead and buy one without stuffing around, there are way more Irwin spinnakers on the Mozzies than any other make. You can just order one from Linsay and get it in the post. Last I heard they are $850.