I recomend you buy a new one and give it a try. But the likelyhood of multiple bad alternators? Are the techs sure its not something else causing premature failure of the alternators?
For my alternators and starters, there is this great rebuilder in town that i use, i have had him rebuild my stuff and never had a problem since on a couple of my mbs. and hes cheap too, like $30-$50.00. you cant even buy a rebuild at that price. But i dont have any contacts in TX.