I've changed the plugs on all my cars (domestic and foreign) over the years and have never bought plugs from the OEM. I just take an old plug with me down to my local auto parts store and match the numbers or I buy them online and have them shipped to my house. As long as you match the make and number of the plug, you will be fine. Also, it's not a hard job, just have to remove some items (air inlet, plug coils) to get to the plugs.
Also, I have never noticed a performance increase due to new plugs unless I've waited way too long and the car is running rough because the plug gap got too large on the old plugs.