Not in NY, but bmrparts.com specializes in reconditioned used parts. I've bought from them and they ship.

 

I'd recommend to buy genuine BMW parts for these underbody panels. I bought a replacement (for #5) that was made of different material (plastic) and it got soft and sagged due to engine heat and was scrapping the road and barely hanging on three screws.

UNDERRIDE PROTECTION Genuine BMW part

 

That article is exactly 11 yrs old. a lot of things have changed since 11 yrs,

 

11 years doesn't change the principals of physics or good aerodynamic design achieved from application of same.

Yes things have changed. More automobile manufacturers have copied BMW's streamlining of the under body flow via belly pans.
In an article published this year GM acknowledged the importance of under body panels in achieving excellent high speed aerodynamic performance in the Camaro ZL1.

I know you leave yours off, believing that you experience better cooling on your transmission. If that were true, piston engine aircraft engines would not have cowlings. Long ago (1920's) aircraft engineers discovered that covering the engine with a cowling to direct the airflow in an efficient stream dramatically improved cooling of the engine as opposed to leaving it hanging out in the air stream. Ninety years hasn't changed this principal of fluid dynamics.

 

Almost all cars have belly pans these days. I leave mine off because I'm too lazy to put it back on. I just started driving it again this week (its been 4 months) but when i have time i'll put it back on. We have good weather besides a rain here and there so I haven't been motivated much to put it on. I care for my vehicles but not to the extent to where i'd worry about putting an underbelly cover on. I'll maintain the car and care for it but I cannot wait for someone to smack into my rear end.

 

Yes, you should keep them on the car. As stated by others, they do several things. One important is improving air flow which then creates less turbulence. Sometimes you can hear and other times you may feel it. I am actually at a supplier since yesterday in Akron that make these type of panels. They are now producing the LWRT which is a light weight fiberglass breathable material. It doesn't hold any water and is a fraction of the weight. Maybe you can rivet a small metal sheet on your part then glass it?

 

If I were you I would try any auto body shop that you know especially if you know somebody that works at the BMW autobody shop they can definitely hook you up with the used piece specific to your car

 

Also I would never buy that piece new, I'd rather attempt to fabricate that peace piece before buying a new one Rather paying them a premium for it. Hell it'd be cheaper to use a 3-D printer to print it out Instead of buying from the dealer

 

I ended up buying the 2 pcs from someone on craigslist for 75 shipped.