uk home office
Note the home office has been split into two seperate organisations (bbc news article), one to deal with security and the other for justice.
These guys are the ones who make the final decision, the Home Office site is full of information, maybe too much sometimes. They cannot advise you on who should help you with your application.
You can though download the forms yourself if you want but remember if you application is refused you will have to find a representitive to deal with your appeal. Most OISC registered companies will charge the same amount for your appeal as they would charge you for doing it properly the first time.
However the onesI have spoken to will do any appeal for free as they will not tkae the case on unless they are confident on it being successful.
The choice is yours, however the simplest and most time effective is getting someone who is registered withh the OISC and knows what they are doing to work on you new visa,