Hello,
There is a quest for capturing a sabino horse but I thought that the capturing was random so isn't it possible i may never capture a sabino horse? Is there a way to make it a for sure thing or do i just have to keep trying and trying and hope that a sabino is captured?

There's no sure way to get a sabino, but if you capture at the beach instead of the forest, there is a 25% chance of catching a patterned horse, and if you use a capture a patterned horse item you will always capture a patterned horse. Both of those options will increase the chance that you capture a sabino.

You can get a capture pass for "Capture a Patterned Horse"

and you can capture at places other then the jungle if you want to try your luck elsewhere. However, quests are money-sinks. Most of them, you spend more money trying to accomplish them then you get from completing them