The Gizmo2 has a PCI/mSATA interface. Low cost SSDs can be had by scouring [ebay]. I went through two ~$10 listings, the first one was DOA. A concern highlighted here [Gizmo2: Installing SSD and Ubuntu] is that "non-standard" mSATA drives need board modification (removal of a resistor) or removal of a trace from the SSD.
After receiving the drive, the difficult part is mounting it to the board. As has been noted before, the board doesn't ship with mounting screws for an mSATA drive. In my workspace, we had some M2 screws, nuts, and spacers from this kit: [here]. The spacer size is not listed, but from measurements with a pair of calipers, the size is 6mm on the axis of the cylinder, 4mm outer diameter, and clearance for M2 screws (~2.07mm) inner diameter. The metal bolts have a head with a diameter of 3.36mm, the nuts are 3.87mm across the parallel flats.
On McMaster, similar bolts and nuts could be bought in nylon. Part No. 95280A110 is a M2 bolt with a head diameter of 3.8mm, 93800A300 is a nylon M2 nut with a width of 4mm across the parallel flats, and 1.5mm thick. This means that you can stack 4 to get a 6mm shim without buying the Robotis kit. The McMaster approach is not massively cheaper than the Robotis approach, as the nuts are ~$10 and the bolts are ~$8. You do get 100 nuts and bolts for that price, though.