Your best bet is to go to a greenhouse specialist (look up Greenhouse Equipment and Supplies in the Yellow Pages or search under “greenhouse supplies” on the Internet), where you can get both durable products and lots of free advice.
Do it yourself
Jacobs Greenhouse, based in Delhi, Ontario, offers a seven- by nine-foot, aluminum frame, tempered glass greenhouse kit, which includes vents and a shade system. You could install greenhouse bubble wrap (yes, just like the packing material) for the winter only, and at least have clear glass over three seasons.
Lexis Greenhouses, based in Ottawa, Ontario, sells the insulation for $1 per linear foot (the roll is four feet wide). Apply it on the inside of either glass or polycarbonate structures.
Jacobs also carries a nine- by 17-foot, glass greenhouse with a rigid frame that can be installed without a foundation.
Plants actually grow better under a translucent covering than a transparent one: nearly as much light gets in, but it is diffused, reaching foliage more evenly without burning it. Among common “rigid” wall materials (a bit of a misnomer, since all are somewhat flexible), double- or triple-wall polycarbonate panelling is the longest lasting (10 years or more).
Lexis offers a kit for a greenhouse measuring eight feet five inches by six feet four inches, with an aluminum frame covered with a twin wall of polycarbonate for $1,260.
The cheapest and easiest material to apply is greenhouse film, which ranges from nearly transparent to translucent and, although it only lasts about four years, is very inexpensive. You can also build a frame yourself and save even more: professional quality, long-lasting, tear-resistant greenhouse film costs about $300 for a 30- by 100-foot roll.
If you have an open porch or veranda, you can convert it into growing space by enclosing it. For best efficiency, insulate the ceiling and the floor, then cover the openings with greenhouse film, or double- or triple-wall polycarbonate, and you'll have growing space you can use all year with the addition of a greenhouse heater.