Southeast of Bravil is the best place to look.
There are some Dragon's Tongue if you follow the road south of Bravil and off to your right a ways. This probable isn't enough to get 20.

For Redwort start at Bravil north gate and swim directly east. Keep going east until you get to the road. Then follow it south until you see cool looking place called Morahame. There is a bunch of Redwort within the ruins outside. You should be able to find all you need.

After that you should be just a little short on Dragon Tongue. Go wander along the Panther River (the river next to Morahame). The stuff seems to grow near bandit camps and ruins, so look for them kind of like land marks.

After you get back into the fold with the Mage guild you can go to the Imperial city Mage place and pick about 5-8 of each from their little garder (Extras incase you happen to steal from them again). While you're there go steal every book in there library as revenge (they don't care if you take the books). Some of the books are extra nifty. Go sell them in the market to make a clean couple thousand too.

Blackforest...

if u go south of bravil follow the path just past (fathis aren's tower ) u can find both there scattered all over in the area but there arent as many redwort flowers u probably will have to search out more or come back 3 days later or so when they grow back