There are many natural remedies to cure stomach ache. I know of one, which I am gonna share below. After googl'ing around, I found more.... honestly, not too sure whether they are effective. It is all depends what type of stomach ache one is having. Know your body well, then you could tell, that is my advice. Stomach ache and stomach cramp are two different conditions, it may ended up needing different remedies.
http://www.myhomeremedies.com/static/stomach-ache.html shares quite a number of home remedies collected from others. Good for reference.
What I could share below is an effective (at least from personal experience) that when I had stomach discomfort, likely due to food contamination and so. It is good to have a cup of warm Ceylon tea, no sugar or milk added. It is handy too, if one does not have active carbon pill on hand. Tea bags could easily get from convenient store or even hotel room's complimentary bar.
Browsing through most published remedies, they do mention tea + honey, tea + mint pretty often. This shows, even if we can't get the accompany herbs or fruits, the remedy can serve us well too.
Plain yogurt may be a good remedy too [remember there was once checking this out for a friend middle of a night].
Disclaimer : This is based upon personal sharing & experience. One needs to know well on his/her own body and condition before consuming such remedy. One may be having gastric while though it is stomach ache, thus pure tea may lead to worsen condition. Take good note and know your body well, is essential!