I work in a garden centre so :
Lavender - one to watch - only fresh ones coming in now are Dutch grown and they are the basic english lavender - better selection available next spring
Roses - new stock due in late October - these will be freshly potted so should have lots of loose compost in the pot
Peonies - anything in a pot will have come in during the spring so check not pot bound and miserable - these next come in as tubers with the summer bulbs that arrive late December or early January
Hellebores - big winter seller so some fresh new stock should be around now and through to early spring
Salvias and other herbaceous perennials - they will be dying down and looking a bit sad, but you may be able to pick up a bargain as many places reduce their perennials to clear through before the winter. Anything clump forming can be split so don't worry if pot bound - they are even more of a bargain. Salvias don't really split very well.
In the new year, most places do a multi-buy deal on 9cm perennials and then 1L from about Feb/March. These are usually great value if you want a lot of plants and are a cheap way to buy groups (3s or 5s ) for impact planting
Hebes & clematis - may well be old stock as tricky to overwinter in a plant area - perhaps better to wait until spring if they look tired and pot bound
Cyclamen - another late season favourite so should be some around - the brightly coloured ones are bedding cyclamen, but we have the hardy ones at work as well
Ferns - the evergreen ones sell quite well over winter so you should find them. The deciduous ones will come under the same advice as perennials
Sorry about the long post. but hope that helps a bit
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