The best paper for fountain pens is 25% cotton, or 100% rag translucent paper. 100% cotton is too hard, it feels like you're writing on plastic. Regular thick high-quality paper is perhaps even better than cotton paper for rigid nibs, because it feels softer. Translucent paper (the type that is used for tracing) is very good, you can write very smoothly on it and if you look at a certain angle as you write, you will see the light reflecting beautifully in wet ink; but it will take a very long time to dry so this is probably inconvenient for writing quick notes in the office - but then again, fountain pens are probably not all that good for quick notes, anyway.
One of the best notepads I found is Ampad small notepad, square ruled, about 5" x 8". It has a very hard back and you can easily write on your lap when you need to. It's also small enough to fit in a large pocket. It has thick paper that I think will do well for fountain pen ink, but I haven't tried it yet. I only tried it with Zebra Sarasa pens, and it works great, of course. These are sold in staples in a package of 4 notepads, 100 pages each, for $7.
The best paper for ballpoint pens is chinese soft rice paper in neat little notepads bound with threads. Their size is also about 5" x 8", and they are very thin, perhaps 20 or 30 pages thick. These can be picked up in chinese paper stores, they come in three varieties, with blank pages (what you probably want), with squares for chinese hyerogliphs, and with the same squares filled in. These are used by chinese children for practice. Ironically, this paper is actually very lousy for the brush ink they're meant for, but it's perfect for ballpoints. These little pads have an amazing antiquated look - especially the cover paper with subtle paper lines and thread bounding. They're almost a work of art. They almost make me want to write with ballpoints all the time. Unfortunately, the only store that sells them is in manhattan and I don't even remember where it is, exactly; it's near bowery, I think you have to go along the canal street until bowery (?) and then go left and it'll be on the right side.
I spent quite a bit of time trying to find these in local Brooklyn chinatown, but nobody has them.