Are these things available at science supply stores?
Yes in quantities of 100 the strips are 3 cents a piece. In larger quantities they are less.
Why are the strips packaged the way they are?
The cover and assembly of the matchbook may be more
expensive than the strips in the book. I do not know NAP/DNP's actual cost.
Certainly the contents of the matchbook is worth less than the shipping.
The difference in price between a booklet with 3 strips and thirty is probably negligible. (maybe a few cents)
But if it had three strips a family of four would have to buy a second kit.
Enough strips are supplied so with normal use a family will not run out of the strips and have more than they can use. I do not think the extra strips are driving up the cost of the kits.
There is a different kit for practitioners that includes more fingerprinting supplies.