Does anyone know a reputable site to buy gil on FFXI. i.e. One that you know for sure will keep you from a ban?

how about guy4game?
Yes, guy4game is a good reputable site which i have used. about 2 years ago, i play ffxi, and need gil, i search many cheap site, but they are terriably,  low delivery, maybe a week,  scammers, and so on.
When i find guy4game, i find they always done a good job, so 2 years, i bought many and many ffxi gil from it. you can just order $30 to test.