For next years fruit and veg, late summer/early autumn is ideal for getting your seeds, cheap and often for FREE!

Peapods - The white powdery covered pods that have only half grown or been caught by the colder nights, etc, are ideal seed material. Simply pick them off, pop them open, into a dish and shake the dish every day for a couple of days (to stop fungus growning on them).
Once they have dried out, place the now dried peas into a brown paper bag (I use the bags I keep from the fruit and veg markets every week) pop them into a drawer and hey presto, next years peapod seeds are all ready and waiting to be planted out again next year.
SAVING - At £2.50 a pack, that's £5 frugally saved, plus the time to go and buy them and driving out to the garden centre etc.

Sweetcorn - I started my sweetcorn too late this year, as I did last year, but when the time came to plant them outside of the greenhouse, the colder nights had already set in. Last year they didn't survive, the same thing will happen if I plant them out this time.
Anyhow, of course they will produce no seeds, the reason they were late planting this year is because there were simply no sweetcorn seeds for sale for over a month.
At £3 a pack, which contained 20 seeds, they are not cheap.
However, at my local market, I can pick up three fresh corn on the cobs, for just £1.
They alone will supply enough seeds for my entire street!
Again, let them dry out on the cobs, pick them off and place them in a dish to dry out. After a couple of days, brown paper bag them up and store them away, ready for next April.

Don't pay over the odds for your seeds next year, use waste fruit and veg that you won't eat this time around, save the seeds, dry them and store them, then put them away.
The more frugal amongst us, spot when fruit and veg is coming to the end of it's season and pick them up cheaply (sometimes for free) at local markets, then dry the seeds ready for next year.