What does your typical daily menu look like? Are you sure you are getting enough calories? I guess you could just be eating plain vegetables and rice or something along those lines, but that is incredibly low sodium for a whole day. Do you like cheese? Cottage cheese has lots of protein and is also relatively high in sodium. And regular cheese too, but that has more calories.

Take a look at this:
http://www.mayoclinic.com/health/sodium/NU00284

It is about trying to reduce sodium intake, but you can just look at some of the numbers it lists for the sodium content of foods and maybe find a few things you could eat that could up your sodium.

The minimum you really need to get to have the benefit is about 500 mg, so you don't have too far to go, really. If you don't like it there's no reason you have to push yourself up to 2,000 mg or anything. But do make sure you are eating enough calories.

Thanks for the link, it's really helpful!

I think I follow a sensible diet nowadays because I eat around 1300-1900 kcal a day but I avoid meat and sweets (I'm the veggie one ^^) and all baked stuff, so that's probably the reason why I don't get enough sodium. I usually eat a lot of whole-grain foods (oatmeal, wheat bran, etc.), a huge amount of fruits and vegetables, and some dairy products too (cottage cheese.. yummy!), and I usually have raisins, nuts, or air-popped  popcorn as a snack. Well, I just don't like to eat all this processed crap.....

It's good to know that the minimum I have to take is 500 mg a day. Gosh, I'm so close! But still I don't know what to change in my diet to keep it healthy, tasty and "high" in sodium.

cottage cheese is generally very high in sodium.  Celery is supposed to be "high" in sodium (relative to the amount you eat).  I presume you eat unsalted nuts?  canned foods are also almost always sodium laden

Oh, I just realized that the cottage cheese I usually eat is very low in sodium, because it's made of milk only (I'm not sure if you ever tried European twarog, it's similar to this curd and looks like that).

Yes, I eat unsalted peanuts..

Please, don't tell me that fresh foods which are high in sodium don't exist.. argh..

Beets have 65 mg for half a cup. Celery 51 mg in one large stalk. One egg has 62 mg. Kale is 30 mg for 1 cup cooked. Turnips have 37 mg per cup ... none of these have major amounts, but since you only have a little way to go you can try adding one or two and that should get you where you need to be.

Would you be willing to drink tomato juice? Types with salt added have 654 mg per cup and might mask the taste. V8 has 480 mg, and I really prefer its taste to regular tomato juice. There's even a "spicy hot" flavor you could try to mask salty flavors. And it has 710 mg per cup.

I think tomato juice, soy sauce and beets will be the best choice for me. Thank you very much for your great help! :)