Everything recommended in the comments so far sounds within reason.

For a dosa griddle alternative to the Lodge Cast Iron skillet (which is fantastic ftr), my folks seem to be pleased by this tawa they’ve had for years.

Idli stands can be a hit or miss - you’re in luck if the racks, knob, and middle anchor align; keep them all together to avoid losing parts - can’t emphasize this enough! Prestige is a well-known, and rather prestigious ;) brand. If you consume dairy, do get quality ghee (neutral flavored oil will also do) and coat each idli well prior to putting the batter in to avoid sticking. I’ve heard that the microwave idli makers are OKAY, but they certainly don’t make the fluffiest, steaming hot idlis we've grown to love and need to be consumed immediately.

If your needs are not urgent and you can spare a trip to the South Bay, might I suggest New India Bazaar for your Indian culinary needs? Their flagship location in Santa Clara has nearly everything under the sun. They also typically carry readymade dosa and idli batter in the refrigerated section.