Like Ray I get all my abraisive stuff from
www.trugrit.com If we were talking belts I would say yes get the best you can especialy the lower grit ceramic belts for grinding bevels (by the way I use VSM ceramic from tru grit) However with sand paper .....most of the time I get it from 2 places HOME DEPOT and AUTO PART STORES....I have tried ordering many kinds of paper including the rhynowet and there is no finish that the expensive paper will give you that you CANT get with cheep stuff the biggest difference is how long the paper last yeh the expensive stuff lasts twice as long but you can buy twice as much of the cheap stuff so it kinda equals out. I think that as a new maker there is SOOO much stuff that costs a lot of money I would get the cheap stuff and save some money for other stuff. (in my opinion) One thing that goes hand and hand with sand paper (at least for me) is a "ultra fine scotch brit pad (grey)" if you bring the knife up to about 800 grit with sand paper then take a piece of one of these and spray it with wd40 and sand BUT ONLY IN 1 DIRECTION not back and forth....you get a very smooth looking satin finish.......Also home depot also has this sand paper that has a rubbery backing instead of paper its good for doing countous where you lay it over the piece and pull back and forth it wont rip....links to both scotch brite pad and the rubbery paper below
Scotch brite pad----http://www.homedepot.com/p/3M-3-7-8-in-x-6-in-x-1-4-in-9-84-cm-x-15-2-cm-x-0-635-Ultra-Fine-Finishing-Hand-Sanding-Pads-2-Pack-7448/301118025
rubbery sand paper ----
http://www.homedepot.com/p/3M-Pro-Gr...-UF4/207178745
they have that in a couple different grits