Reddit mentions of Master Series Nipple Amplifier Enlargement Bulbs with O-Rings

It's not clear whether you want the attention on your breasts continuously or frequently, but there are definitely possibilities either way. I'm assuming that he's able to keep you stimulated when you're together (adding/removing weights from your nipple clamps, slapping your breasts every time you pass each other, etc.), so I won't give any suggestions for that, but for when you're apart:

Continuous attention

• Nipple clamps are fun and all, but hardly an option under clothes throughout a working day. These (for example), on the other hand, are surprisingly effective at stimulating your nipples all day and fit quite happily under clothing.
  
• Instead of biting, maybe try having your breasts struck with a crop or whip, leaving welts. These welts will sting and throb for ages, and can be restimulated by running your fingers over them during the day.
  
• Investigate having a rope breast harness tied, before you go your separate ways in the morning, so that you're bound all day.
  
• Put thumb tacks or something similarly uncomfortable in your bra, so you're hyper-aware of them all day. For bonus, intermittent attention, give them a squeeze whenever anyone's not looking.
  
  Frequent attention
  
  
• Have your Master text you at his discretion during the day with instructions for your breasts - pinch your left nipple hard for 1 minute, hold an ice cube against the nipple until it has completely melted, etc.
  
• Have a set rule that triggers on certain conditions, e.g. every time you go to the bathroom, you have to dig your nails into your breasts and send your Master pictures of the nail marks, or post them to a shared blog.
  
• Get something like tiger balm or capsaicin gel (depending on the desired intensity) and apply it to your nipples whenever the sensation subsides from the previous application.
  
  I hope this short list helps/inspires you to come up with other possibilities :D