Nicholas, Thompson & Morgan have celery seeds. Unless you're desperate to grow from seed, I'd plant seedlings. They will cut your waiting time down enormously.
Celery is also notoriously difficult to grow. If you're a first time grower, I'd start with something a bit more forgiving, maybe runner beans, potatoes, and cherry tomatoes in hanging baskets.
Good thinking, Nicholas. I only ever grew celery once. It takes a very long time, needs well-drained soil, doesn't like too much heat - a big problem here in central Italy - and needs to be kept moist.
It grows well in the Fens - cool and damp - perfect conditions
Perfect for celery, perhaps. I'd rot.