Your orthodontist will normally give you enough packets of elastics to wear them as instructed. Still, elastics do sometimes get lost or spilt, or you may have forgotten to mention at your last appointment that you needed more. In these exceptional situations you need to contact your orthodontist to get more elastics. They are not sold in pharmacies or in shops that patients can easily get to. You may have noticed that the packets have an animal and some numbers printed on them – these correspond to the size and strength of the elastics. You can replace your elastics with another brand only if the numbers on the packet match.
This answer is for general information. Your own case needs an examination by an orthodontist.