I am pretty sure that you cant by replacement straps for polo style breast collars. I would just take it to a leather repairmen and have them make longer straps for it in the same style the ones on it are. Hope I helped!
A harness shop is probably your best bet, but if it's very much too short lengthening the straps might not make it fit correctly, it might still be kind of snug in the chest/ shoulder and the strap that goes over the neck might be too far forward.
If you live close to an Amish community, they always have a local harness maker who can fabricate one for you quickly and at reasonable cost. I had a halter with brass hardware made for my draft cross....superb workmanship and fine leather for @ $25.00.