My guess is your not going to find 3-1/2" stand offs. You can usually get 3" from several sources including Wild Hare, TBM, Desert Aircraft to just name a few. What I usually do is use 3" standoffs and make up the difference with a glued in place 1/2" plywood spacer. I just back up two 1/4" ply squares (1") square and glue them to the firewall.
Another alternative is to go to TBM and order 1/2" standoffs (which they sell) and use them as spacers to get to the 3-1/2" total.
Thanks
Barry