Check with your local shops to see if anybody offers the service. It's always good to keep your business local if you can. A few years back my local shop said he could do it, but didn't advertise it and said that honestly he didn't like to do it.
If not, Buddies offers the service....link was provided in my earlier response. There may be others that offer the service, but since you linked to Buddies in your original post, I assume you feel comfortable with them.
As far as the process with Buddies, you will need to send them your best fitting ball. They will make the thumb mold and send back your ball. They keep the mold and will make you a custom slug whenever you ask.
$25 to make the mold (not sure, but I believe that includes them shipping the ball back to you).
$13 for each custom slug. My local shop charges $10 for a blank slug, so an extra $3 to get it right is pretty good. Of course that doesn't include whatever your shop will charge you for installation. I can't imagine they would charge more than $5 to install it, and if they are drilling the ball for you, they really should just install it for free....but that's just my opinion.