Greg hit the nail on the head. If you go to digikey, you can use their filtering tools to get what you want. Start with "transistor" followed by NPN or PNP, depending on what you want. Then filter on the current you need. I'm sure you'll find some options.

If you're looking for something super simple and don't care about power loss, look for a transistor in a TO-3 package along with a good size heat sink and hardware for mounting.

A SMPS is a good idea if it's in your price range. While these are great, they can be a pain and/or very expensive for the hobbyist. Sometimes digikey will also sell assembled demo boards, which can be a good compromise between cost and efficiency.

6A Current Source
I am able to access a URL in Chrome 36 and IE8 but in Chrome 39 or 40 or Firefox 35 it throws the error:


  Unable to make a secure connection to the server.  This may be a
  problem with the server, or it may be requiring a client
  authentication certificate that you don't have.
  
  Error code: ERR_SSL_PROTOCOL_ERROR}. 


It seems that it is an issue related to the SSL certificate. How can I fix this?
