From the research I did when I got mine, there are no good tankmates. Big fish scare them, little fish they may try to eat, or just catch and kill. Mollies or guppies may work, but they may or may not kill them. Ghost shrimp work and so do nerite snails, both of which I have in my tank for cleanup, but the skippers may decide to eat the shrimp one day. Several sources said a red claw crab may be a good tank mate, which I know nothing about, but I read several people said their skippers ate them when they molted. Soooo IMO get some ghost shrimp or maybe mollies, but that's about it. The tank should focus on the skippers. Anything else pollutes the water up and skippers are messy.
Oh and I think you'd asked about where I got my indian mudskippers - bought them online at livefishdirect.com and had them shipped to my home.
If you're ready to buy the mudskippers and just finding a source, I'd really recommend getting your tank all set up and running if you haven't already. That takes some time and is important to have done and running before they arrive. I would wait to add the marine salt until you find out the conditions they have their skippers in. Mine came from freshwater, which they can do ok in short-term, so I'm slowly bringing up the salinity. That works great for my shrimp and nerites that have been accustomed to freshwater. These mudskippers REALLY appreciate running water so try to create a waterfall with the filter you use. They're most entertaining when you give them what makes them thrive.