They are called "T" pieces and they are available in plastic, brass or stainless.
You will have to nominate your required size when ordering, generally 6 or 8mm for beergas.Take a bit of your line with you when you go shopping.
Plastic ones would be available from your local irrigation supplier or hardware store.
Brass ones would be available from hardware or specialist suppliers.
S/Steel from only a few outlets.
Personally I would not use plastic but if you must I would only use plastic ones on the pressure regulated side of the system.
Some suppliers for the others are;
Mitre 10 Home H/ware etc, for brass,
Blackwoods at Regency Park for brass or stainless.
Barnett Enterprises 2 Croydon Rd Keswick for stainless.
Industrial Fitting Sales 3/12 Tikilara St Regency Park for brass or S/Steel.
I found Barnett's VERY helpful when I went there.
It is one of those places that invite you out the back to have a look on the shelves as opposed to the "shop counter defendant" where everything on there side is top secret and they protect the secret at any cost - even the cost of a sale.
Sorry for going on a bit but I hope this has helped you