You may not even need them at all. MTB tyres tend to just fall off the rim once deflated. My Hybrid is fairly easy as it has 35c tyres on 700c MTB type rims. Yesterday I fitted new tyres on my roadbike, 23c Vittoria Zaffiros swapped like for like then put the old, part worn Zaffiros on to my sons road bike to replace the perished/cracked Kenda 23c tyres he had. All done without tyre levers
. Having said that, I do have a set of plastic tyre levers stashed in my roadbike tool kit but I would have to be tired, cold and wet before I would resort to using them. Nothing wrong with plastic levers as long as they are used properly and are of reasonable quality. In fact, plastic is preferable to metal as it avoids rim damage from ham fisted use.