This is by far the easiest option in the UK.  Plants are widely available at almost all garden centres from April onwards.

Look for sturdy plants with several 'branches' and dark green leaves, reject any which have yellowing leaves. Tomatoes love moisture, so don't purchase any where the compost has dried out, this will slow the plant's growth. 

Size is not really important with tomato plants, small healthy examples should always be preferred over larger not so good ones. Small healthy plants will catch up with larger ones in a matter of weeks. 

The plants can only be planted outside when all danger of frost has passed. Plants bought before this time will need to be kept in a sunny frost free place until the danger has passed. Refer to the main article for more details on planting your tomatoes. The plants can be given a head start if you cover the planting area with a cloche or a sheet of black plastic a month before planting - this will warm the soil by three or four degrees F.