Inexpensive fabric softeners (or natural alternatives)?  

A little bit of vinegar and make sure your rinsing all of the soap out!

I agree about using vinegar. Not only will it neutralize any remaining detergent, it will brighten colors and make the clothes smell good. Is it possible to dry clothes out of doors? If you can that, choose a windy day and your clothes will be soft. Also, I know you said you had a spin dryer, but be sure as much water as is possible is out of the clothes. If all else fails, you can use hair conditioner as a fabric softener. If you have any that you don't care for, or have samples you haven't used, try that.

Laglen is right on about the white vinegar. I use that quite often when I run out of softener or it's not on the list of expenses at the time. You don't need a whole lot, use the same amount you'd use of regular liquid softener. Put it in before the final rinse and there is never any odor from it because it rinses back out. The nice thing with the vinegar is that it removes all the excess soap that clings on the fibers, and you end up with softer wash. (I think it's the excess soap that makes the wash 'hard') I've noticed too that wash seems whiter using the vinegar. I don't have time to bother with waiting to add stuff to the rinse cycle. Instead I bought one of those little plastic laundry balls that holds the softener (or vinegar lol) and when the cycle goes to spin it opens right up and dispenses the contents. I think it was only a buck or two at the supermarket, and was right along with the laundry soaps and stuff.

I use white vinegar instead of fabric softeners. It keeps colors bright, and it rinses out all detergent residue, which means that the clothes are smoother and softer.After rinsing there is no smell. Some people will dilute liquid fabric softener very weak and use a spray bottle and spray their clothes with it. Using a little baking soda in your rinse, if you do not like vinegar, may be what you need to do. I have not done this.

Not only will the white vinegar soften your clothes, but when you are washing brand new colored clothes for the first time, put a cup of white vinegar in the water and it helps set the color in your clothing. I use it when I dye various fabrics so that the dye sets. It will keep your blue jeans and red colored clothing darker for longer and will also keep them from bleeding on your other clothing.