Where to find specific mechs?

Solution 1:

Even after the campaign is finished, some systems are 3 - 3.5 skulls. 3(ish) skull missions should spawn medium mechs. I'm not sure if any of those mechs are favored by a given faction, if so, it may be helpful to take missions in those factions area. For instance, reviewing the entries for the Enforcer and Kintaro at sarna.net shows they were fielded extensively by the Federated Suns, so you might try their space.

Another option is to increase mission variance in the config file. In your game directory there is a file called SimGameConstants.json. In that file you can look for the key "ContractDifficultyVariance" and use a higher value so that you get a wider range of mission difficulties. Lower skulls would have more medium mechs, and increase your odds of finding those specific mechs.

A third option would be to get lucky and find those mechs for sale, but this is far more rare.