Where life starts. If you can't find any make your own by composting, using organic waste, weeds or horse manure from riding schools or your local police station. The art of composting is looked at in the gardening section.

Often you can still find soil beneath tarmac and paving stones.
Look out for building sites, where top soil has been piled up for landscaping.

If there is really not much about or you want to grow tomatoes on concrete, try the splendid Hugel bed technique...

Gather materials such as
branches and wood... for the bottom. This creates a breeding ground for beneficial insects who will speed up decomposition.
twigs, brambles, sticks... to create volume and anchor all the thinner material.
leaves, paper,cardboard, grass, weeds, nettles, hay, and anything similar... piled on top.
And finally mud or soil to cover the top. If there is not much available just use a handful where you have planted seeds or seedlings.

Get the bed at least one foot off the floor.
That's it. This can be used just about anywhere especially carparks. In gardens, mulch the bottom of the Hugel bed to stop weeds coming through.

If you have the chance, find out about the soils history. If it's likely to be contaminated, for example there's a nearby garage, then use non edible plants or fruit trees, or build a Hugel bed on top of it.

Use weeds to tell you what state the soil's in and plant green manures if you are not likely to use it straight away, (although you can combine green manures with planting.)
Thanks to the plasterers in manchester for the studio space @ the plastering company