When we overhauled the Leveled Lists, we also overhauled who sells and deals in MF weapons. You should be able to find MF weapons at the following Vendors/Traders:

• Cricket (Traveling Trader)
• Trudy (Drumlin Diner south of Concord)
• Arturo (Diamond City Surplus)
• K-L-EO (Kill or Be Killed in Goodneighbor)
• Proctor Teagan (BoS Quartermaster)

It is not impossible that a Trader not listed above may have a couple MF weapons, but it is certainly not guaranteed.

If you continue having issues finding weapons at Vendors/Traders, or you simply want a guaranteed place to buy the weapons at, please consider installing Tangent24’s Modern Firearms Dealer mod.