So a number of manufacturers have come up with some pre-mixed painting textures (texture pastes) that you can use either to add in to your paint, or to apply to the canvas before painting.
The most popular forms of texture paste are:
Gesso is sold by many art suppliers and it can very in form and quality. With gesso you really get what you pay for as the cheaper the gesso that you buy, the more watery it is likely to be. Although gesso is quite useful for sizing a canvas or covering over and old painting that you no longer want, it is also useful to provide a small amount of texture under the painting. However, it does not provide very good or deep texture as it is not very thick but can provide some good brush stroke texture under a painting or some subtle lines and marks.
Yes, you can use paint itself as a kind of texture paste to provide texture to your paintings. If you have some left over paint of whatever colour then you can use this as a base. So the lesson is not to throw away any paint that you have left on a palette, whether that be oils or acrylics, but to put it in a sealed pot after you have finished painting and use it for subsequent underpainting.
If you use acrylic paint in particular then you can put this on thickly and leave it to dry overnight if you want to paint over it. You can then paint over it with either acrylics or oil paint.
A number of manufacturers sell products like texture gel, texture paste, texture crystals, impasto gel etc that are very useful in painting. In particular some of these gels remain colourless when added to the paint so that they do not influence or dilute the colour of the paint that you are using. Some of the most useful texture pastes are:
Quite a number of manufacturers make some form of modelling paste, This can be really useful to add think detail to a mixed media painting as it can be put on a canvas in quite a thick application. The upside of modelling paste is that it is easy to use, can be quite light and you can buy it ready made. However, it can be quite expensive, particularly if you want to cover a whole canvas or you have a big painting that needs quite a bit of texture.
Some of the modelling paste you can get is very light so it will not add a lot of extra weight to your painting. Check out some of the different varieties if you want to try something that will add a bit more depth: