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Probably about the best you will find. Only other option is having them custom made at Hanson Rivet, believe it's a 500 minimum.
My measurements for 64963-S as shown on Illustrated Section 230 page 5, 0.144D,0.250L,0.312D HD,0.035T HD,0.101ID 0.137L RECESSFrom Granger, some options, see link for prices:Steel, zinc plated, 1/4 L, pk 100http://www.grainger.com/product/Semi-Tubular-Rivet-1HBP8?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/1HBP7_AS01?$smthumb$Aluminum, 1/4 L, pk 100http://www.grainger.com/product/Semi-Tubular-Rivet-1HCL7?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/1HBP7_AS01?$smthumb$Stainless, 5/16 L, pk 50http://www.grainger.com/product/Semi-Tubular-Rivet-1HEE3?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/1HBP7_AS01?$smthumb$Steel, zinc plated, 5/16 L, pk 100http://www.grainger.com/product/Semi-Tubular-Rivet-1HBP9?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/1HBP7_AS01?$smthumb$I used the rivets in the kit but I needed to fabricate a die to stake and flare the bottom with my rivet tool.Jim
The originals look chrome plated, so stainless would be your best bet.Also, think I mentioned it before, that the originals are tapered from the top to fit the hole in the vent window frame.