For a regular diver making an average dive on a single (12 litre) cylinder will usually be able to make an hour time underwater. This will decrease substantially when diving deeper or in a current, where you quickly will become restricted by your gas supply.
A way to accommodate for this build-up of air consumption is to increase your cylinder size. This by using a 15 litre cylinder or choosing to dive with doubles. However, finding this option on your dive travel or holiday is not always easy.
When we dive with sidemount configuration we usually dive with two cylinders. Not only will this double up your gas supply, but “normal” size cylinders are used everywhere, making it great way to make longer dives on your holiday.
However, diving longer and deeper dives on air is not always a healthy option. If you are making several dives during the day or spend great time at depth on a wreck dive, your nitrogen load might become an issue.
By switching gas in your cylinders to Nitrox or Enriched air we can almost double up the bottom time, due to the lesser nitrogen. Combine this with two cylinder sidemount, you have the perfect recipe for longer, deeper dives on your favourite spot.
Kon-Tiki Thailand 
I like the sidemount idea, I’ve had the issue before too of not being able to find 15L tanks at resorts that have smaller dive operations. What type of BCD do you use to sidemount your tanks?
We use the IST 21 lbs sidemount wing as well as the Hollis SMS 50 during the courses.