It is a great idea to have a mortgage in place before you start looking for a home. All lenders will now do a mortgage application and give you pre- approval. They will not only pre-approve the mortgage but they will lock the interest rate in for up to 90 days. If the rate goes up while you're house hunting it doesn't affect you but if it goes down you can still get the lower rate. This is a smart thing to do. You will know exactly how much house you can afford to buy, you will find out any credit problems that have to be addressed if any and the interest rate is locked in. There is usually no cost to do this so there is no downside just benefits. I highly recommend doing this and also having the lender include a credit check in the pre-approval. A credit check is usually not part of a pre-approval because there is a cost to do a credit check. This cost of a say $10 to you but it's worth it to know if there are any problems that might come up such as a bill you forgot to pay or something else.
Who should you use? I suggest using your own bank first and if they have a problem contact a mortgage broker such as The Mortgage Center who have additional sources of credit that the banks don't.