There are a lot of assets floating about in various repositories but sometimes they can be hard to find. We have a forum dedicated to that job here on the guild where people link to others libraries and put in their own.

But even then its good to have your own set and ability to generate them. You can either draw them, render them or take photos of them. The first needs sheer ability, the second you need the model which you can get in various ways and third one you need the subject and often the location to take the photo top down.

If you have the drawing ability then that is best since you can build up an unlimited library of consistent style icons for your map. But its the most time laborious.

To render them you need the model. A few years ago there were many libraries of free models but the trend has been for the sites to sell models now. Often at not too expensive prices but all the same if you need a lot then you could be spending a large sum to get the models. And then you still need to texture and render them. Texture sites are still common with many having a daily free download and then you have to subscribe to get more or the highest quality textures. I personally like texture ninja but I also have a lot of personal texture photos. Blender is probably best but there are some other 3D apps too. I was using OpenSCad the other day for my last map but Xara and Sketchup are used on the site by people too. If you look at my blog I talk about using photos to create 3D models and Meshroom is now pretty cool but results are wildly variable at this point unless you are really in control about how you take the photos to make the model.

The last is to take photos and use them directly. When you are at the top of a skyscraper then take some photos of cars and street level assets whilst you are up there. Sometimes on a cliff you can get trees from above and Bogie and I were discussing going to garden centres and DIY stores where you have miniature pot plants which resemble their larger counterparts so take some photos of them top down. Then they have floor tiles, statues and architectural features etc. Most product catalogues don't show products top down so you have to do them yourself.

With all of these were talking bitmaps but you can do vector items too if you want to draw them. The usual format though is 32bit PNGs which have 8 bits per R,G & B and another 8 bits of Alpha channel which is the opacity. So these bitmaps have clear backgrounds. Ideally for most maps its best to have the item with a black background and cut out the item with a fade to black around the edge which creates a little depth shadow. The photo needs to be taken without strong side shadow else you cant put the item into your map with incorrect lighting - it just looks wrong.

So there is some skill and art to making the assets and a key thing is just to be aware of the potential to grab some opportunity when you have a camera in a convenient location.