There are lots! My personal Favorite is the Rotary Autocannon Arms for the Hammerhands: https://ironwindmetals.com/index.php/product-listing/product/battletech-20-443racarm/category_pathway-2There are many omni parts and other pieces that can be used to mix and match and make your own variants, I actually sculpted many of them! They are mainly in the new releases category: https://ironwindmetals.com/index.php/categories/cat-battletech/cat-bt-nrAnd lastly, many mechs come with Variant parts right in the blister. Centurion Resculpt, Cougar Resculpt, Catapult C2, Minsk, Hierofalcon, Anzu, Gauntlet, and tons others.
I do alot of kitbashing, if you do a search of my posts, you can see quite a variety of variant mechs.