Every DM dreams of having hordes of miniatures, fully painted, ready to unleash upon hapless players, but time and money tend to be the enemies of us instead. I've broken the page down into 5 categories of expense; Free, Inexpensive, Average, Expensive, and Very Expensive. Personally, I use a combination of these methods. As long as everyone knows what's going on and the players are having fun, then you've met your goal for a tabletop session.
Use literally anything within the size of a 1x1" block to represent the players, monsters, traps, and everything else. You can use Legos, coins, bottle caps, cardboard tokens, etc.
Con: Tend to be symbolic only
It's pretty darn inexpensive to make some fine looking (and customized!) tokens. See my Tokens page for how to create these. There are guides on the web in general. There's nothing at all wrong with how other people do them. I just felt that they lacked a certain final polish.
Pros: The most customizable, durable
Con: Knowledge of image editing
Buy 3d printed minis or print your own. Both can be painted like the pewter ones.
Buy: The Bones line of miniatures by Reaper Miniatures. They're 3d printed and 1/4 - 1/2 the cost of the same mini in pewter ($3-5 vs $6-10). WizKids straddles the line between average and expensive but the quality is impeccable.
Pros: Inexpensive individually, durable
Con: Finer details absent in print
Print: 3D print your own using a 3D printer. There are a ton of free characters and monsters you can print to your heart's content here and here. The only real limit is how finely detailed your printer can go. There are a ton of guides on YouTube and the internet in general. They can be bought (as of 2018) for as little as $200. I would suggest finding an auto-leveling one if you go this route.
Pros: Cost of print, easy to replace if needed
Cons: Cost of a printer, time of prints, not durable
Pewter minis galore! Quest forth, good people, if you have the funds. Each individual mini will run you approximately $5-10, but larger ones such as dragons can go $30 or more.
Pro: Fine details
Cons: Cost, thin parts not durable
Custom minis designed online and shipped to your doorstep at Heroforge.com. Perfectly customize a mini and have it sent to you (in about a month). This is definitely the high-end alt route to hoping you can find the perfect mini for this or that specific humanoid. I only suggest this route with player characters to actually have it made as it's $30 a pop. If it's not a high kill campaign it should be safe for them to invest. You be the judge.
Whether you go this route or not I highly suggest at least checking out the site as it's honestly really fun to play around the character builder and take a screenshot of your creation for portraits on character sheets or for NPCs.
Pros: Highly posable & customizable, durable
*Con: Cost, although they now have the option of selling you the .STL file for $10 so you can 3d print it yourself.