Buy a sewing kit.  Keep it with you in the field.  Your gear will tear, it will rip.  And if you don't have a means to fix that, and the practice to do so -- you're hosed.  

Repairing your kit is way cheaper than buying a replacement.  Don't be mad at me for pointing out the obvious, you know this is good for you.

All sewing kits should include thread, buttons, scissors, needle, and an honest to God thimble.  Recommended to order a couple just in case because these are old stock and some components may be missing.