Depends on if you have the original target head or the Super Spider.
If original, you are now out of luck. Betson was the last distributor supplying them. They discontinued them a couple of months ago. You could try local Arachnid distributors to see if they still have new old stock.
If the Super Spider, they are available from any Arachnid distributor plus the major coin-op parts distributors like Happ, Betson, Competitive Products, Amusements Plus, etc.