I am cleaning out the garage and was wondering if there are any companies that by cores or should I just throw them away?

I doubt you are going to find any company that would buy your cores. However, if you have any original equipment Mopar ones from the late 60's/early 70's, they have value to restorers. Several are worth a pretty penny if they are rebuildable.
Take pictures and list casting numbers in an listing on ebay or in the Moparts ads and maybe you'll make some money!

I will buy all the 67 manual disk brake master cylinder cores you or anyone else can find!

You may want to ask Year One or The Paddock, they may be interested?

Jeff.

I'll take all the bolt down lid drum brake master cylinders you have

hard to get and they dont make new ones

They have small square reservoirs with a single bolt for the lid and are getting hard to find at auto parts stores, now they give you the later style larger reservoir with spring clip for lid. Not correct for restoration type vehicles.