If you want to change the original lower grills you need to take the bumper off.Those are secured from the back.
You can get new ones directly from BMW. Those parts are not so expensive.
If you want you should be able to order parts 2 and 4 at that picture of that link.
http://bmwfans.info/parts/catalog/E9..._bumper_front/
Part numbers
Partially open grid, left 51 11 7 154 551 Price 15.63$
Partially open grid, right 51 11 7 154 552 Price 15.63$
Grid, centre open 51 11 7 154 556 Price 31.26$
Those parts are the European version without that silver plastic and will look like this.Of course without the socket next to my foglight.It's for my engine heater. I'm pretty sure you can order European Bmw parts to USA directly from BMW.
Or if you want the USA version with those silver plastic things it would parts number 3, 5, 7 and 8 on the picture of this link
http://bmwfans.info/parts/catalog/E9..._bumper_front/
USA version lower grills
Partially open grid, left 51 11 7 134 081 price 15.63$
Partially open grid, right 51 11 7 134 082 price 15.63$
Grid, centre open 51 11 7 134 074 price 15.63$
+ those plastic things
Finisher, rod, left 51 11 7 134 093 price 11.33$
Finisher, rod, right 51 11 7 134 094 price 11.33$
Finisher, rod, centre 51 11 7 134 096 price 17.46$
As for the kidney grills you should get gloss black.