My local ACE hardware carries plastic 5/8" 3 way tees which are perfect for t-lines (1/2" id), or Lowe's carries these brass Questpex 3/4" tees that actually measure 5/8" OD 1/2" ID and work great also. Although the 1/2" copper t Diggrr mentions will work, it is a lot easier getting 1/2" hose over a 5/8" barb.
For the cap, just get a 5/8" barb with 3/4" threads and get a threaded hose cap (both in garden hose section at Lowe's or Ace). Run the t-line up, put on the barb, and screw on the cap.