The wobble can be caused by two things; a worn or missing pedestal bushing ( a cylindrical plastic sleeve that fits between the pedestal and the seat slider locking sleeve ) or more likely, the nylon seat slide bushings that fit between the two parts of the seat slider.
There is a post on this forum from a member who found a source for the slider bushing. The first part I mentioned is easy to find, thee later is hard. Search the forum for seat slides, bushings etc and you should be able to fine the thread.
The whole slider mechanism can be replaced assuming you can get the screws out from the old one. All you need is the measurements of the spacing of the four seat screws.
The exact seat is no longer made but there are some models that would fit. Look at Todd seats. Note that the 208 seats are relatively narrow and need to be to allow the seat to rotate around.