I am assuming you mean Kuryakyn ISO grips (or similar), my dealer had the best price on them once I figured shipping in.
Mirrors, probably check places like Dennis Kirk, Cruiser Customizing, etc.
Footpegs, I have no idea as I am running the stock ones.
Gel seat, there was a guy had one for sale on here not long ago, don't know if he still has it. Otherwise, Oneida Suzuki looks like it has the lowest price. Lots of mixed reviews on the gel seat it seems.
Exhaust, MAC Performance, Jardine, Supertrapp, HD stockers. Dennis Kirk is where I bought my MAC, I think they sell Jardine and Supertrapp as well. Although, there are probably better deals to be had out there if you do some Googling.
As far as carb mods, the best offer I can extend to you is to use the
search function on this forum and use the following terms...
carb mod
carburetor
white spacer
jetting
main jet
pilot jet
idle mix
idle
rejet
and there are probably other terms that I haven't thought of since I currently have a splitting headache!

Be sure to set the number of days to search to something like 2000 so you get all the results. Do this as well with the number of results the search engine gives back to you.
Good luck and congrats with your S40. They are good bikes, don't let anyone tell you any different. Tons of things you can do to them to make them unique and totally your personality.
Ride safe!