Strictly speaking you don’t need Slashes with multiple W/ units. They can be connected with i2c cables to whatever you’re using to communicate with your W/ units, presumably Teletype? Though you need the pull-up once there’s about 4 modules in your network.
At any rate, as @ellips_s says, it’s just a board, not a modification to your W/ units. You wouldn’t have to send your current unit in.
AFAIK WR doesn’t sell them independently of sets of multiple W/ bought directly. W/ to W/ communication isn’t implemented yet, so you’d only want/need Slashes at this stage as a convenience to power multiple W/ and communicate with Teletype.
It does look nice for avoiding a rats’ nest of cables. If you’re not up for DIYing the board I’ll bet you could someone here on Lines who would be willing to work with you. The PCBs need to be bought in sets of 3 which means people are likely to have spares. If you paid for the boards and components for 3 for example I would happily build you one and ship it to you.