Avoid any which are labelled "Jumbo oats" They are OK in muesli etc but they don't "cook down" so well as porridge. That said, some normal sized oats do seem to be tougher than others. Longer cooking may soften them.
Like Blondiescot I buy Scotts Porage Oats or Tesco's own brand Scottish Oats (I suspect they come from Scott's factory), but they may not be available where you live.
I cook them in the microwave - one measure of rolled oats to four of water, or water and milk, plus a pinch of salt, stirred then given five minutes on full power. Not stirred again until the five minutes are up - just long enough to make tea or coffee and get the rest of the breakfast ready. They need plenty of space to boil up or they overflow all over the turntable.