Both the US and UK are not into D/E subs-the Brits last built their conventional subs early in the 90s which were later sold off to Canada. France, Germany and a host of smaller European nations are the only builders now.
India’s submarine programme still needs extensive foreign assistance and it’s foreign policy remains risk-averse other than the occasional armed exercise with Japan or Vietnam. Taiwan is a different ballgame given Chinese sensitivities. Japan would have been a good bet but their foreign policy is about the same as India’s. The Taiwanese lost an opportunity of lifetime when they signed big arms deals with France in the 90s for fighters and frigates; they could easily gotten submarine tech.
The Indians don't need to build the submarines, just purchase Russian subs and resell them to the Taiwanese. The Taiwanese should be able to reverse engineer parts. Ditto for other weapon systems.
They then have a good argument to answer Chinese protests with demands of their own concerning the Indian Ocean.