Look at Dougram/Robotech/Macross/Mospeada/Crusher Joe/etc Models for the early designs, getting pretty scarce now as the shows were broadcast many years ago and the kits are old and show it.
Fasa copied/used a lot of japanese mecha designs and later had to drop them to avoid getting sued.
Horizon did a 1/35 Madcat in Vinyl(OOP), Armorcast offers 1/60 Resin kits of the Atlas, Madcat, Vulture and accessories.
(got all the above except the Atlas).
If you want to search for a specific design/mech you will also need to know the non-battletech names that they were released under (i.e. the names used in the original Anime shows).