West Isles primary offices are listed as Farsky Station over Aberdeen, though they are also registered "couriers" in League space, ostensibly operating out of Neo Athens on Calysto.
In reality, if you go to either of these places and try to find an actual office or place of business, you will be sorely disappointed. West Isles operates as deep in the shadows as d'Ardenne and his crew can get. Having a physical office defeats the point quite a bit.
Pro tip from an old spy: if you need to find West Isles, try Mengxiang Zhi Can on Farsky. They've got pretty good dumplings, fantastic drinks, and a truly unexpected view for a station. If you absolutely have to find Walker d'Ardenne and his crew, try there. If they think you could use their services, they'll find you.
West Isles, as a corporate front, has existed for roughly 150 years. It began as little more than a shell company in League space for tax dodging purposes, but sadly, the executives of the company using it had very little idea of how to use a shell company. As a result, when the parent company got hit with the mother of all tax audits (courtesy of a friendly, anonymous whistleblower), they were found to owe something on the order of a few hundred million in back taxes. Seeing as they weren't the most profitable company in the galaxy (hence the desire for tax dodging shell companies), they had very little choice but to sell the company for parts. One interested party snapped up the West Isles shell along with everything connected to it. Within a few years, everything owned by the company had been sold off at a profit and the company was "re-tooled" into a "courier" service within League space.
This entire sequence of events was put into motion by Adrian Phelps, the thief who trained Ralph Marks. West Isles became something of an inheritance as it serves as a convenient money laundering operation and a handy way to ensure that there is a perfectly legitimate paper trail for expenses. For over a century, a lineage of thieves have been pulling jobs across the settled galaxy and claiming tax deductions on every single one.
Prior to his death in 2684, Ralph Marks passed the ownership of West Isles over to Walker d'Ardenne. d'Ardenne didn't quite know what to do with it at first, but after Marks' death and meeting Janson, an idea formed pretty quick. Why just claim that West Isles was an actual company? Why not operate it as a company? d'Ardenne purchased three Hazard Hawk 60 ships, hired some small crews to run them, and turned West Isles into a perfectly legit courier company.
A perfectly legit courier company that just so happens to have a fourth crew made up of the best people d'Ardenne could get his hands on. Besides the completely off the books pay that the Chilkoot crew pulls in, they are also paid a fairly standard salary complete with tax deductions and pension withholdings.