I like cover crops where that works out....just some of my beds work out for that. I think it is too late now for you.

I suggest adding very finely mulched leaves, leaf compost, and/or well rotted manure. I also like to work in a bit those things and make some ridges for early crops next year...to get the soil warmer and dryer early.

Check out freecycle if you are anywhere near a farming area. You might luck out on goat/sheep/horse poop. Also the Animal control might be a good place to get rabbit/chicken/guinne pig poop/bedding!

Also, if you have coffee shops in your area, often they will save grounds for you (I usually provide a bucket with my name on it and a plastic bag) . Do NOT forget to pick it up! LOL You will loose your coffee ground privleges!

Yes, if you're overwintering the bed, I'd just mix in mulched leaves, if you have any around. Maybe it's past that time for you? They will be thoroughly composted by the time you're ready to plant in the spring. Manure would be fine as well and again, I'm not sure I'd worry about it being composted. Bulking up soil with organics is much easier to do at the end of a growing season than at the beginning of one. If you can turn the bed once or twice during the winter, that would be especially good.

I also cover unused beds with cardboard to prevent weeds over the winter (we are surrounded by horse fields) after putting down the manure/leaves etc., and poke holes for water to get through. Nancy