I have a Western Digital Scorpio Blue Serial ATA that is 9.5mm and this slots perfectly with the Motherboard laptop SATA connectors. However the new hard disk 2.5" is 6.8mm high, so how do i get this to fit with the same slot/caddy/motherboard?
Most of the time it just works.
You don't necessarily need a spacer with a caddy. The 7mm drives have the same screwholes, and the connector's at the position. Simply install the thinner drive exactly the same way as the old drive and it'll fit. I've installed those drives on older thinkpads as upgrades and they work great.
If it slots into the motherboard, topside facing up and the connector on the motherboard, it'll work too. Not too sure if I've seen that arrangement.
I suspect a spacer would be needed if you had a weird design where the drive simply slotted in next to the motherboard, and the connector position relied on the distance between the top of the drive and connector to fit, rather than the standard mounting holes, or if the drive needed to be installed 'upside down' for some reason.
Typically this should not be an issue, and I wouldn't buy a spacer until I was sure I needed it.
I'm assuming that this drive will actually fit fine and connect, but what you are wanting is some kind of spacer to properly align the drive and stop it moving around.
I've found a spacer on scan.co.uk that looks promising but as the only description is "SSD spacer to increase height from 7mm to 9.5mm" I don't know if it has adhesive to hold it to the drive or if it is plastic or foam.
Here is another spacer.
It appears that some drives actually come with spacers, but that's not as helpful after you've already bought a drive.