If you are referring to residential units--single family houses, townhouses or condos, those will be listed for sale as regular listings. They may or may not be occupied with tenants. You can buy any home and rent it out. Often listings with tenants are below rented at below market rates and if they have a lease, you won't be able to raise the rent until the lease expires. They generally need to be renovated to bring in higher rents and good tenants.
I own several rental properties--if you own properties in good condition they will rent easily. I've renovated all of them and they rent well.
If you are looking at multi-unit complexes, some are listed with residential real estate agents, others are listed with commercial agents.
Before you start working with an agent, think about how you plan to finance your purchase and rehab, and locations that you want. Are you looking for cash flow or building equity? It makes a difference in what you choose to buy.