I bought one from the supermarket in a 1 litre pot pre Christmas

Potted up to to a bucket sized container prior to spring flowering and it put on a good show of flowers and growth.

Not sure about long term though as the pot was an interim measure until I found the right spot in the beds.

Although, I'm sure if you keep it fed it will be fine.

There are also patio container varieties of these available now in some of the catalogues I've had through.

J Parkers do a patio pot version (dwarf) in their autumn catalogue. You will have to wait until the autumn to buy from them. Grows to 1.2 to 1.5 metre tall. I wouldn't recommend trying to grow the normal type in a pot.

Forsythia always does best when grown in the garden. It tends to rot very quickly if it is allowed to get too wet in a pot. Also why would you grow a shrub in a pot which is just sticks for three or four months of the year?