I use wet/dry sandpaper (wet usually) on my models (largely 1/72 aircraft) which can be bought at most lumber yards, home improvement stores (Lowe's, Home Depot etc), automotive supply stores and hardwares. The coarsest grade I usually use is 360 grit (although I have been known to use 220 grit on a real problem area) and depending on how fine a finish I want work my way down to 1800 grit. Just remember the smaller the number the coarser the grit, the larger the number, the finer the grit.