I use the keypad numbers to buy stuff. These are my binds, just put them in your userconfig.cfg (or make one) or autoexec.cfg files in your steam/steamapps/username/counter-strike source\cfg directory:
bind "kp_end" "buy ak47; buy m4a1"
bind "kp_downarrow" "buy ak47; buy m4a1; buy vesthelm; buy vest; buy hegrenade; buy defuser; buy flashbang; buy flashbang"
bind "kp_leftarrow" "buy hegrenade"
bind "kp_5" "buy flashbang"
bind "kp_rightarrow" "buy smokegrenade"
bind "kp_home" "buy vesthelm; buy vest"
bind "kp_pgdn" "buy deagle"
The big bind buys what you can based on your cash..you can change the order or add/remove stuff
I have q to select HE grenade and E for flashbang..handy for quickly throwing a nade. I change primary/secondary weapon with the mouse scroll wheel. My binds for those are:
bind q "use weapon_hegrenade"
bind e "use weapon_flashbang"
You can read about making buy scripts here:
http://counterstrikesourcetactics.blogspot.com/2008/02/counter-strike-source-binding-guide.html