Inspired by nature
There are lots of reasons why making the most of what's in season is a good idea. It's certainly possible to get all sorts of flowers year-round, but the more out of sync you are with nature, the more airmiles they rack up and the more they have been chemically treated to cope with the journey. (Those flowers that stay in bud and rot without ever developing? Someone's been heavy-handed with the silver nitrate.)
Go with what's seasonal and you get fresher, better quality, flowers that retain their scent; you can mix them with your own finds from the garden, and your money goes further. It also means you have the option of sourcing your flowers from local growers who can offer varieties of country flowers which the big importers just can't handle and that, in turn, means you can create something that is definitely not standard.
If you have time to plan ahead, and a bit of space, you can have a go at growing flowers yourself - Sarah Raven and Georgie Newbury have both written really useful books on making your own cutting garden. But a word of caution, flowers are no respecters of calendar dates and it's always best to have a back-up plan in case the sweet peas aren't playing ball. Foliage might be easier to rely on...