Instructions
Shop with a specialty Lego supplier like BrickLink, where individual users can buy, sell and trade specific pieces, if you want to find a lot of individual droids. Buying the droids individually is much cheaper than buying a set that comes with droids.
Expect to pay between $1 and $2 for each droid, depending on its new or used quality. It may take time for you to build your droid army given the financial restrictions, so be patient.
Build your own droids using yellow pieces or gray and brown pieces. Before you build, organize your pieces and assess which pieces you have the most of.
Build your droids using the pieces you have the most of, rather than the pieces that make the most realistic droids. If you end up making basic droids out of a few 2 by 2 or 1 by 2 blocks, do make one or two droids that look as realistic as you can make them (using cylinders and other rounded pieces) and use these as the commanders.
Create terrain for your droid army to make a battlefield to display them. Some hedges or bushes, small lakes or streams and ruined buildings for the droids to hide behind are all suitable battlefield additions.
Create some Jedi to fight your droid army. Even if you don't have the specific characters sold by Lego, you can create a Jedi simply by giving a Lego person a light saber. If you don't have any actual Lego light sabers, use what you have, like neon antennae or other specialty pieces.