Try 914a&p.com or in the member vendors section in here.
IIRC, all USA cars had to have tinted safety glass windshields - along with a few other items like the front round side marker lights, all of which were grouped under an option" which shows up on your Certificate of Authenticity (COA) as something like "USA Equipment".
They also had another option called "Tinted glass all around" on the COA's, which meant that both the side windows (driv. & pass.) were tinted along with the above noted tinted windshield already under the USA eqpt. (other countries got the windshield included under this option IIRC, if it wasn't required like in the USA back then).
So if you're trying to keep your 914 as it was originally equipped, either look at the original window sticker, dealer invoice/sales receipt, etc. &/or order a COA from Porsche Cars No. America (PCNA) & hope that their info is complete & correct (they have had some problems with staff reading the older codes, etc. on many COA's, including mine). Go online to the Porsche website, USA, & under the Classics section of Customer Service it the info & form for the COA's. While you're on there in Classics, also download their free pdf parts catalog FYI.
Ooops - make that the Canuck version of everything - I just noticed the B.C. in your sig.!
You can get a discount as a Porsche Club of America (PCA) member $20 I think, plus Porsche will discount it $20 if you send them a copy of your window sticker. PCA is $42 a year & you get 2 magazines monthly (national & local), 20% discounts at Porsche dealers about 2-4 times a year (one is running now), plus $20 off your COA - in addition to the various events, etc. Not a bad deal.
.... & hope this helps!