I have one but it's running in my secondary system, if you had posted this a few months ago and I had seen it you could have bought mine reasonably cheap. They hold their value pretty well though so you would be looking at an easy $240+ on ebay. Try looking on r/hardwareswap for them, even if there aren't any listed maybe someone there could point you to someone who has one.
Hello @strin6w1z4rd , you have definitely posted in the correct segment of the forum
Unfortunately the majority of Intel's higher/mid tier processors utilise 20 PCI-E lanes but thankfully they're all Gen 3.0 with that said any motherboard vendor manufacturing/supporting the LGA 1151 will split the 20 lanes 8* by 8* for either Crossfire or SLI configurations.
Best guess is to choose from;
Asus's MAXIMUS VIII GENE
Gigabyte's GA-Z170MX-Gaming 5
Couldn't really find an M-ATX form factor motherboard from MSI which is pretty unusual since they did produce great small form factor motherboards.
Regarding how much you want to spend is up to you but I will inform you that there are some motherboards with support for DDR3(L) specifically so you can save yourself so money instead of investing in a newer more expensive product such as DDR4 unless you feel like the cost is necessary.