Also take a shifty look in the tall, wheeled isle bins that often stand near the fresh dairy product isles while the staff are stacking and clearing the shelves. You may spy excellent quality clear bags being thrown away, some of substantial sises that you can put 2 or 3 full size trays in. Also, look out for clear plastic wedge shaped lids that go over 1 dozen packs of some yoghert type products. There is one in particular that is absilutely the same size as a seed trey, and it has a ready made "frosting" effect to diffuse the light a little. I'd tell you what brand it was but I have never actualy asked what it came off. There are also ideal multi-pot stands to be had, to lift 6 or 12 plant pots at a time (just make a mental note of the diamiter of your favourite plant pot bottoms and you are laughing). The staff are usualy only too delighted to ferret out a particularly usefull looking item if you explain it is for gardening purposes. If any bag, lid or multi-carton you see dosn't look pristine when you get it home wash it with a very mild detergent and rince it well. The inside of bags can be done in the same way. If I come across a good, tough transparent one I will wash it carfully and re-use it several times. Sometimes I use detergent but if they are getting greenish or they smell I use a little bit (a puff at most) of anti-bacterial surface cleaner spreyed into the bag, rub it arround a bit and then rince out the bag thoroughly. I've never had any problems with mould after doing this. Finally, experiment carfully with freezer bags, which do have the massive advantage of being re-sealable. Most of them are perfectly Microwave prooff. So if you put a little water in the bottom and sit the bag, open and upright in the microwave it will steralise itself in a very short time. Check that they don't melt - some say microwave safe on the lable just to help you out. Also, goes without saying really, carfull lifing the hot, steamy bag out. A rince, followed by microwave will provide a recycled sterile bag if you don't want to have to keep buying new ones.
Lastly, if you can find a book or website that gives a general and reasonably comprehensive guide for different plants that need germination in light, or those that need shade, or darkness to germinate please let me know because a chart or lists would be very useful. I could put up in my potting shed for quick reference. Maybe Gardeners World Magazine could publish one???