Hi Tom, sorry to hear about your disappointing trip, I have done that sort of trip myself.
Now that said what weights of card stock should one start collecting?
Tom there are all sorts of displays that are put up at retail stores that are for promotions these are usually fairly short lived in the store and are a good source. As far as the thickness goes try to get as wide variety as you can, you don't need a large volume, just a wide variety of cards.
If you have a picture framing shop locally ask them for some of the scraps that are really too small for framing, this card is pretty high quality, you can even buy a sheet and get them to cut it in half or into a quarter which makes it much easier to handle and deal with.
I use an old concertina file box to hold different thicknesses of card in, and even very small bits get saved until I am overflowing then cull the smaller bits and keep the larger ones.
I have found that different types of packaging supplies very different grades of card. Boxes of tea have a pretty fine grained hard card whilst Kellogs Cornflakes boxes are coarse. Some Pizza boxes are really tough as well. When gluing the printed package type cards together to form thicker stock I use some fine sandpaper to scratch up the inked side.
If you can purchase a simple dial caliper it is really useful for testing the thickness of not just card but all sorts of materials and things.
The great thing with working with inexpensive supplies is that when you make mistakes there is no difficulty in making a new one without breaking the bank.
When working with corrugated card it is a good idea also to cross laminate the card which helps to prevent warping.
I just measured the thickness of the different cards that I used on the simple ton side dings and they are:
for the sheets that covered the foam picture framing matt board card .053 inches or 1.3mm
the white card on the surface .031 inches or 1/32nd of and inch or .78mm
and further a Mcain Pizza box is .015 inches just a hair ove 1/64th of an inch or .45mm
A pepsi cola box .015 inches
A manilla file folder is .010 inches or .25mm
A generic kleenex box .014 inches or .37mm
The back cover of a Robert Bateman sketch book .062 inches or 1/16th of an inch or 1.56 mm
The back cover of an Omni sketchbook .080 inches or 2.04 mm
These are as close a measurement that I could get with the different calipers that I have, so if anyone does the mathematical conversions then these might change very slightly but not enough to worry about.
working with card is not precision engineering but the general thicknesses are fairly consistent.
I find that the matt board about as heavy as I like to cut with a Number 11 exacto or equivalent blade, I am fortunate in that I found a perfectly good paper cutter at the dump...go figure!
A paper crimper is great for making corrugated sheets.
The final choice of thicknesses will really be your own preference in what you feel comfortable cutting.
I hope this is useful.