Antlers
I use the ones made by Hysteria Machine; they’re light and (I find) relatively comfy, so I can keep them on all weekend without too much trouble.
For larger, more impressive antlers I’d go with Mandala
Whichever you choose, practice wearing them before the event - I tend to pop mine on while I’m pottering around the house - so you get used to compensating for them; particularly with large antlers, you’ll find your physicality changing slightly, which can be useful for getting into the character, and you’ll need to factor them in when going through doors etc. If you use a shoulder-bag or satchel it’s worth practicing putting it on / taking it off with the antlers on - the first few times are a glorious clumsy tangle of straps and hair and antler. I’m in the League, so I’ve also gotten quite adept at donning and removing masks with my antlers in place.
It’s also possibly worth thinking about how you might need to adapt your costume to accommodate their presence, for example chainmail that’s split down the front and buckled back together is probably more practical than a shirt you’d have to pull on over them. Hats and hoods are An Adventure.