The mail order company I use has Crocosmia advertised in their Spring-Summer catalogue. Spring planting for summer flowering.

Down by the river today there were fresh new leaf blades pushing up from the shingle, having easily survived the floods of winter... (Crocosmia is often weedy here) They need good sunlight to do well. Too much shade and you'll get mostly leaves.

You could buy in autumn - so long as your soil drains well, and allows for the development of the long 'string of beads' of the old corms, plus the runners that spread out to increase the clump before forming a new corm. Check with local plant swaps and farmers markets as someone might well have them on offer.

You would probably be buying something with old flower stems and seed knobs. Just allow the foliage to die back naturally - and provide moderate water while it settles in.