The Laundromat is quite rare in GTA IV as it Can't be found Commonly in the Streets. If you already have one, Drive around for a bit and more Laundromats will Spawn or Drive around. You Can Obtain the Laundromat in Three Missions in GTA IV:
1. Hung-Out to Dry for Vlad - If you try to take it before going inside the Laundromat Store, you will Spook the Owner of the Laundromat Van and the Mission will Fail. Despite that though, you can take the Van to your Safehouse and Save it.
2. Luck of the Irish for Elizabeta Torres and Patrick Mcreary - After this Mission is Completed, the Laundromat Van is yours for the taking.
3. Meltdown for Ray Boccino - The Van is Seen During a Car-Chase During this Mission. The Player May be able to take it, but you'll have to be quick as you enter the Vehicle gives more time for the Enemy to escape. Sometimes taking this Vehicle can lead to Mission-Failure.
This Vehicle Can also be Seen and Driven in TLAD, they are Seen twice in the game in Two-Missions:
1. End of Chapter for Jim Fitzgerald - Brian's Crew can be Seen Driving them in Port-Tudor. After Defeating Most of his Crew, you can take any Laundromat that's Still Working and Functional. If all the Laundromat Vans are Destroyed, then you Can't Obtain them (Obviously). Once you Defeat the Remaining-Enemies, the Mission is Complete. Once you Complete the Mission, go to your Safehouse and Save the Vehicle (If you have One). When Chasing the Remaining-Enemies, they are using Bikes, not Laundromats.
2. Get-Lost! - For Congressman Tom Stubbs - The Final Mission in TLAD - Several Laundromat Vans Can be Seen Parked in the Alderney State Correctional Facility but they are locked and Cannot be taken. Johnny won't be able to Break the Windows. It's the Same for the Prison-Bus in the Same Mission. The Two Vehicles are Properly Mean't to make the Prison-Facility Stand-Out either before or During the Prison-Break.
The Laundromat Van Does not Return in either TBoGT, CW nor GTA V.