There is nothing special about Truvia, it is just a brand name for one particular stevia formulation - there are many others. If you are interested in trying stevia then there are several things that you should be aware of.
Firstly, there are some health concerns. Personally, having read several papers on this, I think that these are based upon quite poor science - but this is something you will have to make up your own mind about. Secondly, because of these concerns, it is illegal to sell stevia in the UK or in any other EU country. It is, however, sold and widely used in most of the rest of the world and it is perfectly legal to import it for your own use. There has been lots more discussion of this in
another thread on stevia - I suggest that you read that thoroughly before deciding to go this route.
If you do want to give it a go, then the only way you are likely to get it in the UK is to order it online. There are a number of web sites that will sell it to you (Google is your friend!), or you can usually find it on e-Bay. There are many formulations available under many different brand names. Truvia is a sugar-like powder that is made by mixing stevia extract with polyols. Various stevia formulations are manufactured like this, and I doubt that there is much to choose between them. However, since polyols aren't themselves a hugely good idea you might be better off going for a formulation that doesn't use this approach.