I don't know of any public website that will allow consumers to specifically search for land contracts. As Realtors, we pay for monthly access to our MLS (Multiple Listing Service) to search for homes. It has more robust search functions which allow us to search specific items like land contracts, but is not available to the public.
The market in the Grand Rapids area has improved drastically over the last year and a half. There is a very low inventory of homes for sale right now. Since land contracts are usually not favored by sellers, this means two things for someone who wants to purchase with a land contract 1) you will have very limited options on what homes you can choose from, and 2) you will not have a good negotiating position.
My question to you is why do you want a land contract? Usually it is because a person cannot qualify for a traditional loan. My advice is do whatever you can to qualify for a traditional loan so you have as many options available as possible. In the meantime, you can have a Realtor search for homes that are willing to consider a land contract. With land contracts, the seller will expect a large down payment, usually a minimum of 10-20% of the purchase price.