Be careful with the specifics of your question. What do you mean by "cold?" What specific temperature parameters? There are many medications which must be kept below a certain temperature, but may also not be frozen and thus may not be inserted into certain types of packs.
There are great options for all of these, but it is critical for you to post the exact parameters you seek. Also, bear in mind that if you have a 24-hour series of scheduled flights, you are really looking for cooling for at least 48 hours to account for disruptions.