It’s a bit early for most perennial plants, especially if you are buying from garden centres or nurseries.A lot of perennials will be dormant at this time of year , they don’t look very attractive as plants so aren’t really available. I would start looking in March or April.
If you know exactly what you want then buying plants online is usually pretty reliable, though remember to factor in the postage. I have bought good stuff at fair prices from eBay in the past. often online plants are sold slightly smaller than garden centre plants which can bring the price down, though it can mean that your new bed might look a bit empty for a bit while the plants catch up.
Normally during the spring and summer there are lots of open gardens to visit - though things will probably be different this year. Google Yellow Book Open Garden scheme to find ones near you, the owners very often sell plants (and home made cakes) that they have grown themselves. Often at reasonable prices.
Plant stalls on markets are usually good value. If you live near east London then visit Columbia Road Flower Market on a Sunday morning.
School fairs , when they start up again usually have plant stalls, as do bigger boot fairs and church fetes. Some are better than others, sometimes it is all spider plants and leggy tomato seedlings , other times real treasures. It’s all luck.