Finding insurance for a commercial van is not really any different to a car. Most of the big, well known insurance companies will do vans as well as cars. Many insurance brokers, who can search for the best quote across many different providers, will deal with van insurance as well as cars.
There are also specialist providers for commercial vehicles, including vans, who advertise on the internet and in specialist magazines about trucks and vans. The main things that you have to insure against are the same as a car but you may need to consider specific extras like insurance for your equipment, things that you carry that belong to other people, and public liability cover.