I guess you al buy tomatoes in some for or another. Why not collect your sown seed from the fruit that you eat. It is so easy really.
Take a tomato on a bread board that is ripe or over ripe, cut it up. Take a clean honey jar or similar of water about half full an place the tomatoes pieces in the water. Put the lid on and give it a good shake. Leave on a shelf in the house and when you are going by give it a shake. When all the seeds have sunk to the bottom of the water in the jar decant the top out. When you remove the lid it will be a bit smelly but it is really worth the effort. Decant the top out of the jar carefully so that you dont lose the seed. As the bits of tomato puulp fall out the the jar so does some water come out. Put fresh water in and continue till only the seed is left in the bottom of the jar. take a flat plate and some kitchen paper and pour the seed out on to this. Leave for a day or so to dry out, store in a air tight jar till you are ready to plant.
You can use this method on all pulpy fruits: Eg..... figs, strawberries and any of the berries that you enjoy, tomatoes,