i need somebody with a mac to create a custom firmware 4.2.1 with cydia on it (to restore my iPhone 3GS)

i used redsnow to install the ipad baseband and to jailbreak my iphone, the phone worked for 5 minutes, then crashed and everytime i use redsnow to try to boot tethered again doesn't work

i found one custom 4.2.1 looking in google but that one doesn't have cydia on it

i can't come back to 4.1 because i don't have shsh

THANKS