Mechwarrior singles are a very good suggestion. I got rid of most of my extras, but if you find anyone with a collection, they've probably got some great industrial mechs (these were the worst in the game) that they will give you for free or near-it. They're ideal if you're looking for a model that's going to stand 2-3 inches tall.
If you want something bigger, then Robogear kits are a good bet, you can find them on ebay.
If you want something smaller and cheaper, the Battletech-sized EM4 Mechs give you quite a number of options for less than a buck each.
http://www.em4miniatures.com/cgi-bin/sh000001.pl?WD=mechs&PN=Science_Fiction%2ehtml#aSFP3
Last, and possibly best, the Reaper CAV series has all manner of Mechs and you can buy all the legs separately! There's a huge variety of robot legs to choose from. CAVs tend to be around 2 inches tall, but some are smaller and some are taller.
You should look in the CAV section. Each Mech should have the parts broken down on it's sale page.
https://www.reapermini.com/Miniatures/CAV:StrikeOperations
and the "robot supply" section where old CAV models ended up
https://www.reapermini.com/Miniatures/RobotSupplyDepot
CAV parts will probably be more expensive than the other options I listed, but MUCH cheaper than buying the Forge World kit.
Best of Luck!