it typically takes 2 to 3 weeks for plants to show sex when first flipped into flowering...
the plant has to stop producing veg hormones, and start producing flower hormones.. so it can take a while for that to happen.. this is the delay in sexing...
if you veg a plant long enough, usually about 4 weeks or more, they usually will show preflowers.. so one way to avoid the sexing during flowering wait is to veg your plants out until you see preflowers... also, that is a sign the plant is mature (along with alternating nodes) and shows the plant is ready to flower to its fullest potential..
for future reference, when going from veg to flower, giving your plants 24 to 36 hours of straight dark before starting 12/12 can help speed sexing up by a 7 to 10 days, and also take some time off of flowering itself.. its too late to do that now.. but remember it for your next grow...
be patient.. you should see them soon...
as far as nutes, here are the top brands (in my humble opinion) on the market:
general hydroponics flora 3 part system... they also have a 2 part system if you dont want to mess with the micro formula.. and plenty of additives to choose from as well.. i personally use this line..
fox farms has a great line of products.. again a 2 or 3 part system and additives..
technafloras BC line is a great line... same deal with them...
advanced nutes... ive seen amazing results with this brand.. havent used it personally.. but do know fellow growers that do and swear by it.. tons of options with this line as well...
all the brands also carry organic nutes if you want to go the organic route..