100 Yen shops - what & where

I read about these shops.
Is it only Daisho or other names as well ?
Can I get all soughts of traveling needs like toileteries ? How about toiletries bag, towels(for onsen in case not provided) and disposable underwear ?
Are they easily found in Osaka and Tokyo ? Which locations of in these cities have more or a bigger store ?
Do they sell only 100 yen stuffs or more ?; I will not expect towel to be 100 yen.

Is it only Daisho or other names as well ?

There are several chain stores (e.g. Daiso, 100 Yen Plaza, Silk, etc.), and many independent ones as well.

Can I get all soughts of traveling needs like toileteries ? How about toiletries bag, towels(for onsen in case not provided) and disposable underwear ?

Yes

Are they easily found in Osaka and Tokyo ?

Yes

Which locations of in these cities have more or a bigger store ?

The Daiso on Takeshitadori in Harajuku is quite popular with foreign tourists.

Do they sell only 100 yen stuffs or more ?;

Some sell more expensive items. They will be clearly marked with a price and the cashier will also make sure you know when an item is more than 100 yen.

I will not expect towel to be 100 yen.

Towels are indeed 100 yen. They aren't usually of the highest quality, but will be sufficient for travelling/onsen.

100 Yen stores are similar to the "Dollar stores" we have in North America (though the later are likely to be an imitation of the former).
All sorts of things are fairly good, even though they don't cost much.

All over the place. My favorite was at the Akihabara station, Yodabashi-facing street level exit. Got a very nice tie for 100 Yen along with a few other kniknacks...