Remember that millions of people bought them as an investment. At some point the owners will die, or move to a smaller house or get sick of them. So the stamps will end up in the hands of dealers. Most dealers that I know have literally tens of thousands of pounds worth of mint stamps, for which there is very little demand. So they use them for postage.
So what's the best way to buy them. Firstly, remember that presentation packs and year packs are probably nicer to own than lots of individual stamps. And they will certainly be worth more. So just ring up a stamp dealer and explain that you would like to buy a large number of packs. Ask him if he'd be prepared to sell them at face value. He almost certainly will! Tell him that you'd be happy to come and collect them or he can post them out. And tell him the years that you want. If you tell him, for example, that you'd like all the year packs between 1970 and 1980. he'll probably have them in stock.
And it's all relatively cheap. The photo shows a 1986 presentation pack commemorating 'Industry Year' (face value £1.02) - a 1981 collectors pack (face value around £5) and a very pretty 1994 presentation book (face value around £11). The 1994 book has a catalogue value of £55, but I regularly see dealers stripping out the stamps and throwing the books in the bin. Dealers see very little value in the packs and books, so if you want them you should get a very good deal.
Incidentally, I have only mentioned British stamps. But I suspect that the same applies to stamps in most other countries. There is very little demand for modern stamps, but they are lovely and - if you're canny - you can collect them very cheaply.