Yes you can buy a second hand slicer from a butcher but you need to be careful if you do. Often they are getting shot of them because they are simply too big and have gone for more compact models. My first slicer I bought was a 12" Berkel from a butcher - it cuts like a dream but it takes up so much space and takes two people to lift and move it
The one I use most now is my Buffalo 250 mm. I bought this new from NextDayCatering. They have a good selection there
When selecting your slicer be sure to check how easy it is to take apart and clean after you have used it. The blade needs to be easily removable and also safe to remove - most professional grade slicers come with a removal tool so if you are buying one second hand make sure that it is still with the machine. You should also check out how heavy the parts are and how easy it is to take apart for cleaning before you decide on the model you are going to buy. The smaller domestic slicers designed for home use usually come fully apart and most of the food contact components are designed to fit in a dish washer. The professional ones dont.
Because of the difficulty to completely clean these machines between use, in professional food preparation establishments (in the UK at least) it is not permitted to slice raw meats (like bacon) and ready to eat foods (like cooked ham or cheese) on the same slicer.
I hope this helps