I've always bought my seed potatoes in late Feb to chit and plant in March/April. I plant the first earlies first, then seconds and finally main crop. They all go in together but the first earlies are harvested now, the second during July/August while the main crop take about 5 months to grow and are dug up during Sewptember before the cold weather.
Most garden centres have finished selling the seed potatoes since most people have planted ther's by ow, but as you say, you might get some online.
If not pop some courgette plants in the space left by the spuds, they're quick and easy or some brocolli for the winter.