"I suppose..." and "I think..." are similar but they are not always used the same.
When I say "I suppose so" usually it's in response to something I'm not eager to admit or confront.
Like, if someone said to me "You should go to the doctor about that." I might say, "I suppose so."
"I suppose" in normal usage (in the US at least) is closer to "I guess" than "I think".
"I think" is more for situations where you aren't sure of something. Like, "I think I will be busy today." (You aren't sure.)
"I suppose I will be busy today" is more like you *know* you will be busy but are somehow resigning yourself to it.