I have been into fountain pens for a while now and have learned a bit and I remember how daunting it was at first. I also didn't really have anyone to ask, either. Let's take a peek at the selection and make this less scary.
Rhodia PaperRhodia seems to be the most popular in my opinion. Most ink reviews shown online are done on Rhodia paper, which is a very nice, quality paper. Rhodia paper is 80g/m2(grams per square meter) or approx 20lb paper. What makes Rhodia special is that is has a thin ceramic coating on the paper making it very smooth and somewhat ink resistant, which makes great fountain pen paper as it helps bring out the shading qualities in ink. It will have longer dry times than "normal" paper used in the US but that is part of the fountain pen charm.
ClairefontaineClairefontaine paper is 90g/m2 which is roughly 24lb paper. Clairefontaine is not ceramic coated like Rhodia but it is very smooth regardless. Smooth paper is very nice to write on with a fountain pen but can be troublesome with an overly polished nib and can lead to skipping, sometimes referred to as baby's bottom. Some of these terms used in the fountain pen world make you wonder.
FabrianoThis paper is my personal favorite and measures in between Rhodia/Clairefontaine at 85g/m2 which I would assume would be around 22lb paper. Fabriano does not have a ceramic coating nor does it have a smoothness to it. It has something called tooth which is described as rougher textured paper. Fabriano has some tooth but it is not dramatic, just enough to still have a great writing experience as well as eliminating that baby bottom issue. It also has ink resistance and really makes shading pop with shading inks.
Tomoe RiverOk, this stuff is magical and defies all logic with paper. It is 52g/m2 but is super thin...like tracing paper thin. This voodoo paper is phenomenal and handles fountain pen ink like a thicker paper. Ink looks amazing on it, it is super smooth to write on and it takes quite a bit to make the ink bleed through to the other side. It's magic I tell ya. The only downside is that it is the most pricey of the bunch but it is definitely worth it.
Is that all?No, not at all. I am not going to list all of the papers, just the most prevalent and popular ones.
Do I have to buy fancy paper to use a fountain pen?Nope, not at all but there are some things to consider. Using standard US based paper, this stuff is thirsty when using fountain pen ink and the wider the nib used on a pen, the worse off you will be. Feathering, spread and bleed through are the main culprits and frustrating factors here. Generally using fine or extra fine nibs are the way to go on standard papers and ink that is not runny or lubricted is preferred. I do like Noodler's X-Feather for this as it is made for the less than ideal paper.
I know I have thrown a lot at you here and that is only because there is a ton of choices out there and it gets confusing. There is nothing wrong with trying out different papers to see which you prefer more but hopefully now, you have a little more ammunition to make a better choice. Watch out for any paper cuts!