Is there anywhere that still sells SD3 kits?
I can't seem to find very much.
I'm especially interested in Chocolate Cake Drums 'Metalhead' kit, but I haven't been able to find it anywhere other than Steam - and there doesn't seem to be any way to download it from there.
Any kind of Metal-style kit would be great though.

Not explicitly metal, but Platinum Samples still have six or seven bundles in their arsenal for Session Drummer 3. The one kit that maybe comes closest to what you’re looking for might be the Evil Joe Barresi Kit 1 for $15. And maybe you can process the kick and snare to come even closer.
 
http://www.platinumsample...EvilJoeBarresiKit1.php
 
Or, do yourself a favour and buy XLN Audio Addictive Drums 2 Custom containing 3 ADpaks, 3 MIDIpaks and 3 Kitpiece Paks of your choice. This bundle is still available at half the regular price at AudioDeluxe for just $75 (using code SUMMER2017). Maybe a combination of the ADpaks Metal, Fairfax Vol.1 and Black Velvet or Studio Rock, together with the MIDIpaks Metal Songs, Diabolic and Heavy Rock might be a working solution. And for the three kit pieces I think XLN Audio recommends DW Solid Piccolo Snare, Sonor Designer Snare and DW Woofer Bass Drum for rock and metal.
 
https://www.audiodeluxe.c...dictive-drums-2-custom
https://www.xlnaudio.com/products
 

Also Platinum Samples Rock Legend kits (Gretsch + Zildjans) are great too, I use them a lot,  last time I checked they were at $20