Well nothing is entirely free but here is an idea for the serious gardener....and recycler.
You will need :
1 wheelbarrow for collection
2. a few new composting bins or a large pile in your garden
3. extra worms from wigglywigglers
4. a bit of time
As the tomatoe growing season gets to an end, many people in the UK will be wondering what to do with their used growbags. If you want to get some extra compost for free ( which you can revive by next year), you could print a small leaflet to put through doors within walking distance to offer to collect same ( charge if you want but thats up to you). Do leave your phone number or place where people can drop them off for you to process. You could put a wanted on freecycle too.
You will be getting bags of them so say NO when you have sufficient.
What to do with them :
Some of it can be used as a mulch on your beds, yes its depleted but by the time nature has worked on them with the worms in the ground, it will raise your soil levels. You could plant a green manure crop on the top of it over winter and that will increase fertility.
You can mix it with your current contents of the compost bins or bokashi waste and leave it to work over the next months until spring when you will be able to dig it into your beds.
Alternatively use the above to make your own soil mixture for next spring by adding some possible extras such as the organic fertiliser from wiggly wigglers:
Coir has similar properties to leaf mould and can be used as a substitute.
Sand Adding sand to your mix will increase drainage. For most purposes it's best to choose a gritty grade of sand, with a diameter greater than 5mm.
Perlite is a volcanic mineral that's excellent at providing air spaces within a potting compost. However, it won't hold on to water, and nutrients are quickly washed out. It's ideal for composts that require very free drainage and low fertility.