I just found a spare for my 2015 GLA on eBay, and the tool kit there too.
Here's my post about it on the GLA Forum. The tire is from a 2007 S550 and the tool kit from a 2013 R320, and it all works perfectly for my GLA (which has slightly oversized tires).
If you calculate the diameter of the tires on your car, you can figure out which "donut" spares from other model years will give you that diameter and then you can shop for spare tires for those models. So, I got full genuine MB parts (spare, jack, tool kit, compressor, tow hooks, lug nuts) for under $400. I'm sure you can do it even cheaper, but I wanted the complete tire kit with the compressor just for fun. If shopping for a used spare, make sure you get the lug nuts with it so you don't have to find those. Odds are the lugs from your full-size wheels won't fit on the donut. Have fun, good luck.