I don't believe they sell the parts here, but if they do, I would certainly recommend shopping here, the service I got when I had an issue of my own making was outstanding. OWC does sell them. http://eshop.macsales.com/item/OWC/DIYIM...
I added an SSD to mine using the kit sold here Mac Mini Dual Drive Kit turned the two into a fusion drive. It is MUCH faster than it was with just the 5400 rpm drive. The fastest would be to just add a large SSD. You could either add it as a second drive and then leave the other for archived docs and such, or you could replace the drive with an SSD. Read the instructions on this site carefully, this is not a simple process.
It's pretty well hidden, but they are assuming that the installed stock drive will be in the lower position. You can see this on step 37.