You can find longer tubes at the places that supply the contractor trade. In my area, there's a company called Free (uses a logo that looks just like the Ford logo script but in red) that has the stuff that is longer. Sonotube is a brand name of a concrete form tube so that the contractor can pour a round post from concrete.
As to focal ratios, the important thing is to look at the focal length instead. This determines the Field Of View (FOV) of the scope for any particular EyePiece (EP) and longer focal lengths (fl) means that you can use longer fl EPs for the same magnification of the scope. This means that you get better eye relief (nice if you wear glasses) and also allows for more magnification without going to stupidly short fl EPs. The downside to longer FL scopes is that they also tend to be narrower FOV scopes - witness that my 200" fl (F45) refractor uses 3" diameter EPs (homemade ones) and has trouble seeing more than .6deg. as a FOV. This means that there's a lot of stuff up there that I can't see all of due to the narrow FOV. What is nice is that the eye relief is great on my EPs tho even though they are a primitive design.