This could be more trouble than it's worth, but I used to work retail at various malls and none of them had a recycling program. We threw away a heap of bubble wrap and stuff every week with shipments. If you want to, you could go by the stores (ones that sell breakable stuff, but also book stores) and see if they would be willing to save some for you. Have to pick it up on schedule, though! One problem might be that the pieces could be small and you'd be doing a lot of taping.

I used 1/2" bubble wrap in my rion 8 x24. I bought it at UPS store for 250 ft. roll for $35. It's not free but not expensive either.

Here is one picture with bubble wrap.

Thanks for replies. Does th bubble wrap really make a difference?

I bought a clear solar pool cover for my 6x8 and wrap the whole thing in it for winter (except the door, of course). I have not tracked energy used, but I do feel it makes a difference. I cut the leftover material into strips which I hung, overlapping, inside the door and that, too, makes a difference. Working out there on a sunny but cold day, I can leave the sliding door open and the strips do a good job of keeping the heat in while allowing me to enter and exit without having to constantly mess with the door. (Especially handy if I have my hands full.) The pool cover was about $100, but we store it away when not in use and so far have gotten 7 winters' use out of it.

I also insulated the north wall and the north 1/3 of the east and west walls with 1" thick, foil faced, foam board insulation. Above, on the lower portion of the north ceiling, I place a strip of 2' wide Reflectix insulation. These materials reflect light, as well as heat, back into the greenhouse.

The first winter, I learned how important it is to put a draft barrier between ground and wall. I had put in a hardware cloth barrier to keep rodents out, but neglected the draft barrier. Pulled the gravel back and improvised that first winter, but the next fall installed a proper barrier.

Of course, these things need not be done all at once. The materials are quite strong so will last for years, allowing you to do it in phases, if you wish.

Attaching the solar pool cover: we've trimmed it so it fits nicely over the greenhouse, tape the rear end closed and tape the front, on either side of the door, to the frame and/or polycarbonate. It is secured at the bottom on the long sides with 3 wooden strips on each side. A drawing would be best, but don't have that capability so here goes:

Two are secured parallel, horizontally, to the 2x10 treated lumber base with screws, and they stay on all year. They are spaced so as to allow the third strip to be attached between them, sandwiching the pool cover beneath. This has held up well through several winters, including some storms, and is fairly easy to attach and remove.