Metalminiatures are always classy, and generally look the best. There arelots of sources for these; TheMiniatures Pageis a good place to find out more.
Cheap plasticminiatures can be had from U.S. Toy. Go hereand search for "spaceships" and "space cruisers." They don't have thequality of metal ships, but they're dirt cheap and perfect forG.O.B.S.!
Paperminiatures are available in vast amounts; most are free or veryreasonably priced. You will need a printer that can handle cardstock(110 lb. paper; check your printer's specs), or you'll need to pastethe paper onto stiffer stock. Some of these are not all that easy tobuild, but the final results are impressive. Be aware that while thedownloads are cheap (or even free), paper and ink costs can add upfaster than you think. A good place to search for paper miniaturesources is Paperworlds.com.
Space, ofcourse, doesn't care what ships look like, and neither doesG.O.B.S.! You can build very passable spaceships out ofplastic wall anchors (sometimes called "mollies"), which areavailable for pennies at hardware stores and discount marts. Beprepared to trim off flash with a good craft razor! Plastic wallanchors are made to be functional, not aesthetic.
For aparticularly unusual "organic" fleet, I recommend sea shells, whichyou can buy in large quantities for practically nothing. Party supplyplaces often have these, in various sizes. Many of the little pointedspiral jobs just beg to be turned into starfighters!
Use yourimagination. You can turn practically anything into a spaceshipminiature; craft beads can be glued together, old model parts can beconverted; I've included one race that is meant to have toy cars andtrucks as its fleet. Check with the women in your life (or, if youare a woman, peruse your make-up stand) for old lipstick tubes, etc.A lot of these have unusual shapes that might serve as the basis fora spaceship. Think about the cap on a toothpaste tube-- doesn't itlook like a nozzle? Check out the innards of some "click" styleball-point pens; those little plastic pieces have some unusual shapeswhich might be incorporated into a spaceship (or simply used astorpedoes). With a little glue, paint and some creative thinking, youcan make a spacefleet that's uniquely your own.
Good luck, andgood fleet hunting!