Getting sponsored in the twenty-first century means getting online and using one of the sites spefically set up to collect sponsorship for fundraisers, claim gift aid and pass it on to you charity. But they are not all the same.
The three main sites that I came across are:
There are comparisons of these three here and on Wikipedia here. In the end I opted to use the Virgin Money Giving service as it is clearly badged as “not-for-profit” unlike it’s main competitor Justgiving.