A Bolt keeps working itself loose on my Ford Transit 54plate?

Have had problems with the bolt coming loose and losing gear selection,this week however the bolt came out completely,i lost all gears,managed to roll it safely to the side of the road,i found the bolt in the bottom of the engine, How can i stop this from Happening again?is there a better bolt available now?and if so where to purchase it?Hoping someone can help me out in this.