Adventurous visitors to the former Soviet Union can now go on trips they would never have gotten from the old Intourist state agency. In Moscow alone, there are some 200 small, private travel agencies offering everything from white-water rafting in Siberia to glacier skiing in the ultra-rugged Pamir mountains of Central Asia. The most unusual excursion, from Felix Travel, is to one of Stalin's gulags for political prisoners on the White Sea near Archangel. The price--a bargain $120.