I usually get my Ficus and Umbrella plants from B&Q. They do several different sizes too. A few weeks back I went in to get Lily a new Umbrella plant and they had no Ficus or Umbrellas at all, then the next week I went in and they had loads!! Maybe they are awaiting a delivery.
i wouldnt recommend buying a ficus or umberella plant from b&q or homebase as they tend to use pesticides on their plants.
Also its just starting to come into the season for these plants to be shelved in gardening centres.
A good place to look is a notcutts.
Also ebay have a couple of ficus benjamina for sale at the moment.
Have you thought of alternative plants?
Pothos or Dracena are small plants, which should be ideal for the size you require.
If all else fails and you feel you have to purchase a plant from b&q i suggest you give the plant a good wipe over from leaf to root.