Like the NPC modified Dubsta and the Lifeguard Granger you need to be in an identical vehicle in order to spawn one.
If you have a friend who has one then all you have to do is ask him to drive to the Bugstars warehouse in Elysian Island and ignore the rest of this guide.

If not you need to start the Bugstars Gear Part 1 Casino heist Big Con prep mission.
I suggest you do this in an invite only session.
Well all Casino heist prep missions should be done in invite only sessions...
Go to the location Lester gave you and you will find a Bugstars Burrito with a few armed goons guarding it.

The van Lester told you to steal is a mission vehicle which means it can not be stored in your garage which also means you will need to spawn another van using the one you just stole.
You will find one at the Bugstars warehouse in Elysian Island:

When you arrive at the warehouse you will find a bunch of Bugstars Burritos parked inside it.
They are the vans you will be able to store inside your garage and modify at the LSC.

As you may already know you can not store vehicles in your garage while on an active Casino heist prep mission.
Now you need to find a way to fail the ongoing mission.
Easiest way is to blow up the van Lester told you to steal.
It would require 3 sticky bombs to blow it up (Rockstar really beefed it up in case people would try to do the preps in a public lobby but we are smarter than that, right?)

Before you drive your new van into one of your garages you may want to purchase insurance for it.
The full coverage package cost over 56K but it is still way cheaper than if you buy it from Warstock Cache & Carry.


Congratulations!
You are now a proud owner of the Bugstars Burrito :D