They're not hardy plants, so don't buy till late April or May - can be planted outside when all risk of frost is past (end of May). Garden centres often stock them around this time, otherwise source plants online. Just not now.

I think you may have a problem finding plants. Although this is a perennial in Mexico in the UK it is grown as an annual with the seed being sown time to sow the seed is late winter or early spring.

I have seen them in garden centres, and believe it or not they were described as hardy! They were also over a tenner!
Seriously, buy the seed now and sow on a window sill. The seed germinates better if you pour water over them that was boiled ten minutes earlier, and then leave to soak for 24hrs.

Over a tenner for a plant that will only last a year? Sounds like a rip off to me... As Meanie says seed sown now will produce good plants and you'll have more than one plant, heck you could sell some to neighbours for a few pounds!