Mr. Stripey
Heirloom Tomato in the US

Starting heirloom tomatoes from seed is the way to go, seeing as you rarely find good plug plants. Get your seeds going indoors about 6-8 weeks before your last frost date, using seed trays or small pots filled with good potting mix. Sow them about a quarter inch (6mm) deep. For folks in Zone 5, March or April is your window, same for Zone 6 in February or March. Zone 7 growers can start February or March, and if you're in Zone 8 or 9, January or February is ideal. Keep them warm, around 70-75°F (21-24°C), and give them plenty of light.

Once your seedlings have their first proper leaves, called true leaves, gently prick them out and pot them up into individual 3-4 inch (7-10cm) pots. Keep them growing strong indoors or in a greenhouse. Before planting out, you'll need to harden them off for about a week, gradually getting them used to the outdoors. Plant them in the ground or in large containers once all danger of frost has passed, making sure they're a good 2-3 feet (60-90cm) apart for decent airflow. Strong staking is a must, like I say, these can get big and heavy.