Of course you don't actually have to use a beer can. Practically any kind of can or bottle will do. Try 24 or 32 ounce juice or soup cans or a similar sized mason jar. A local warehouse store probably has dozens of options you can try. If, however, you are a purist, try a Fosters beer can or anyone of the other 24 ounce beers. This is just about the perfect size for a 15 pound turkey. If you want a larger turkey you'll need to move to something in the 32 ounce range.
The original recipe for beer can chicken started out with by telling you to dispose of the top 1/3 or 1/2 of beer. How you dispose of this amount of beer is up to you. The best strategy is actually pour out all the beer into another container. You need to cut off the top off the can and that's easier to do if the can is empty. With the top off the can you can now add your beer and seasonings that will flavor and moisten the turkey as it cooks. If you are already using some kind of seasoning rub, marinade or baste for your turkey use the same seasonings in the can. This will add to the flavors without conflicting.
Why this works so well is that first of all you are adding a source of moisture to the turkey that keeps it from drying out. Second you are adding beer. Now, more than the fact that beer is good, the yeast and malt in beer react with the turkey, particularly the skin, making it thin and crispy while the meat remains juicy. Garland Rome suggested to me using crab boil, which works really well.
The trick to getting this to work right is making sure that the turkey stands up right. Now with a good 20 ounces of liquid in the can the weight is on your side and the turkey should anchor well. But as the liquid is the can steams away the can will lose its weight and the turkey might topple over. This would be bad. One trick is to cut about three inches of the bony end of the turkey at of the bottom of the cavity. This will allow the turkey to sit a little lower and give it a more stable base. It also reduces the overall height of the turkey when standing on end.
Which brings me to the big point about beer can turkey. Whether you are grilling, smoking or oven roasting your bird it is important for the turkey to fit with at least two inches of space between the top of the standing turkey and the top of whatever you are cooking the turkey in. If you've cut the bony end off the back of the turkey it will sit lower, but before you try a beer can turkey make sure it fits.
If you are cooking a large turkey it is going to take some time. During this cooking time the can might just boil dry so it is important to put as much liquid as you can in the can without it spilling over while you are getting it set up. You best bet is to set the can on the cooking surface you are going to use and then carefully lowering the turkey onto the can. You might need an assistant for this. Take your time and make sure you watch the can carefully. And when the turkey is safely set to cook, wash everything. No point being sick for the next week.
So consider a great solution to the age old problem of a dried out turkey and have a little beer on me.