I have an mSATA enclosure that came free with an mSATA SSD that I bought from MyDigitalDiscount. USB only, and really poor build quality, but they do indeed exist.

If you would like to quickly add more storage via USB or access data on M.2 SSD drives, you may be interested in a new piece of kit launched by CHIEFTEC in the form of the CEB-M2C M.2 NVME SSD ...

The versatile USB to mSATA converter supports all models of Raspberry Pi as well as other development boards and is now available to purchase for around $32.