I don't know if this will help, but here's what a buddy of mine did:

He needed a small piece for one of BOSE's speaker stands. He went to one of their stores and asked them if he could order one. They just went into the back of the store, grabbed the part, and gave it to him.

If you've got any BOSE outlet stores near you, try just waking into one to see what you can get.

I also regularly see older BOSE speakers at a pawn shop I've shopped at. Maybe find an older pair at a pawn shop or somewhere that won't cost that much and pry off the labels for your car speakers.

__________________

Current:
2014 VW Tiguan SEL 4Motion 43,000 miles.

2016 Hyundai Santa Fe Sport (wife's).

Past:
2006 Jetta TDI 135,970 miles. Sold Nov. '13.
1995 E-320 Special Edition. 220,200 miles. Sold Sept. '07.
1987 190-E 16 valve. 153,000 miles. Sold Feb. '06.
1980 300-D 225,000 miles. Donated to the National Kidney Foundation.
1980 240-D manual, 297,500 miles. Totaled by inattentive driver.