You should never use a metal tool on powdered iron or ferrite tuning slugs, adjustment tools for them are usually some type of plastic. You should be able to find these online. The better the fit, the better it will work.
Heating with something like a hair dryer might help loosen them. If the coil form is plastic or phenolic you might try a little bit of liquid lubricant, some coil forms are made of paper and may be damaged by it though.
Once the slugs are broken they will be very difficult to get out, you generally have to use something to break them up into smaller pieces to get them out.
We used to use a little bit of cornstarch to lubricate them.
As for a source of replacements, you'll just have to do some shopping. There are a lot of places out there selling vintage parts. You may have to buy some surplus coils with the same size form just to get the slugs.