On any established wood there will be seedlings in the understorey waiting for something else to come down, providing an opportunity to head for the light above. These seedlings will be adapted to the exact conditions of the wood, don't cost anything, and are probably appropriate natives. The 'natural regeneration' approach or using seed from your own existing trees has many advantages, apart from maybe taking a bit longer than purchasing and planting nursery grown trees.
If on the other hand, you want to introduce additional species which aren't there, you'll obviously have to buy them, unless you bump into someone with a few going spare. Do think of wildlife when planting new trees, whereas Eucalyptus is perhaps one of the fastest growing and best fuel woods, it has virtually no wildlife benefits, we don't have koalas or other species that can feed on it. Natives are always best for wildlife, they are an integral evolved part of the ecosystem.
This lot near me seem very competitive with a good range, although suppliers nearer to 'up North will have trees better suited to your local climate (Eg Yorkshire Artic Tundra, rather than the Devon tropics).
http://www.perriehale.co.uk/