You say you want to use a particular Lender/Mortgage company, who has told you your next step is to seek out a Realtor. That's very good advice. However, bear in mind that some experienced Realtors with prior financial backgrounds, etc) also have a good idea of what Type of Loan, and even what kind of Mortgage company may be a 'Good Fit' based on the bigger financial picture and your very unique housing needs. Hopefully the Realtor and Lender can work together to recommend the right kind(s) of programs or be flexible enough to fit your unique needs.
For example as I get to know you, I find out that you'd be better off with a zero money down type of loan, you'd probably be better off using a lender who actually has (or is willing) to give you a zero money down loan.Or as I get to know you I find you are Military, and you are renting your house in North Carolina and the lender you chose to use say's they 'can help you' but there are certain requirements you will have to meet and certain repercussions that impact you in ways you had not anticipated. So getting Pre-Approved by the 'right' lender is one of the many ways an EXPERIENCED REALTOR can help.
Sometimes it's also best to see an Experienced Realtor first. I can save you a lot of time for example if you tell me the Income you make is 'not typical' wages, and you want to use that income for making mortgage payments. There are some Lenders who will go along with that, and there are some who may not go along, or if they do, your interest rate may go up. That doesn't have to happen if you are guided to see the right lender from the beginning. And even if you shop two or even three different lenders (with the guidanceand with the assistance of a Realtor who has done this before) you are better off! I can tell you about this in great detail. (And your credit score does NOT go up if you see them all within a 30 day period!) And this isn't uncommon. Believe me there are more 'unique' ways of making income these days than you realize.