Apart from parsnips which only last the current year (purportedly), most seeds will last at least 2 years and many (particularly if foil sealed) very much more. Keeping them in a dry sealed environment will maximise their life. I have found the limited range of seeds at Lidl reliable (5 for £) or the slightly wider range at Wilko (usually 75p to £1.50) equally good. For particular varieties, other seed companies charge you £1 to £3.99 usually for fairly limited quantities and extra postage.