That's one of the less popular 11.5g slugged plastic offerings from the heart of the poker boom 15-20 years ago. Matching chips may be challenging to find because the most common 11.5g chips from then were "dice chips". Switch to dice chips you'll never run out.
If you do manage to find these, demand that they be cheap. I mean, nobody on this website would pay a penny for these, so don't get overcharged when you find them. Similar 11.5g slugged chips are abundant, so I would try to pay $10 per 500 chips, and defintely not more than $20.