The site cgtextures has lots of good textures. If you don't pay, you can download up to 15MB per day for free, and there are some sizes of texture you can't get, usually the biggest sizes, which are not as useful for games, rather for cg rendering. Usually you can still get the ones around 1024x1024, but bigger than that are the ones you can't get for free. They have pretty cheap memberships as well if you need things bigger and want to lift the daily 15MB limit.

As far as making textures yourself, I recommend you maybe learn to do it yourself. The filters in software like the GIMP help, but sometimes you can get away with 3d renders in Blender as well, though you get mixed results depending on how good you are at producing models.

I don't know off the top of my head, but there are books you can buy that teach how to make textures. There was one that I had checked out from a friend that had some good techniques. Some tricks were to take photos of real world objects and combine them to get other things, using different layering techniques, masking, coloring, and other things in the GIMP or photoshop. You could take a sand texture and cover it with a masked cage or something like it, turning grayish the sand(maybe blurring it too) to maybe get a sort of sci-fi-ish metal. Different ways to combine textures can get good results. You can also use one textures colors turned to only grays to create a layer to recolor the layers below, for example to darken or lighten, to modify saturation, or even to shift hues. This, combined with cloud filters for masks(or other filters as well, or even combinations) can get interesting results too.

ShadowFlar3 has a good point. The site cgtextures.com does allow commercial usage, as long as the textures are integrated into the project, and not easily accessible alone. A game is a fine case of this. Many other websites on the other hand indeed don't allow more than personal use.

The other point about the textures fitting together is also valid, but depending on how easily you can either learn or hire someone to do proper art for your specific case, it may be easier to grab the free textures and modify. I think there is enough variety that they can be found and modified to fit well enough for most games, but it won't be as easy as simply downloading them most likely. On the other hand, it may be easier than learning to create the textures yourself, depending on what software you know, how quick you learn, etc....

Hi,

Once you spend a few hundred hours at it, making textures will take a fraction of the time that it did when you began. I am amazed myself at how quickly I can whip together a nice texture for something, sometimes in a matter of minutes, that used to take a lot of hours.

I use GIMP for most things and actually could get by with only using it for 95% of textures, as for rasterized rendered ones. Inkscape is often used by me for vector graphics, some of which are later converted to rasterized JPG, BMP, and so forth. Once you get into designing vector planes in your games, then you will open a whole new level of capability for quality, even if most other surfaces are textures. Eventually, procedural generated surfaces will make your game appearance reach very sophisticated quality level.

In the meantime, continue looking into these things and expanding your knowledge base. I'd focus on layers, including transparent background layers, gradient filters, fonts, and lean toward more Alpha Channel inclusion in your game after a few months, maybe only weeks. Down the road you may use Blender or other software to generate procedural surfaces, sometimes in combination with normal and bump maps. All in due time!