My local garden centre is the best place for bulbs round here. They have a 'fill a bag' stand. They work out cheaper than all the deals in B&Q, Wilko etc. I divide one bag into 2 or 3 layers by using layers of paper and put different types of bulbs in the one bag.
I bought some shrubs from J Parker in the past which looked like bargains but turned up very pot bound - not surprising really as the pots were tiny. Wouldn't use them again.
If you want cheap trees, you could consider the Lidl/Aldi/Wilko pound shop fruit trees which they tend to get in Jan/Feb. It might be something of a lottery but at least you can look at the plant in the shop rather than buying mail order and then finding you've been sold a weedy little thing. If buying trees from any other supplier wait until November when you can get them barerooted rather than buying now when they will be pot-grown and may have been hanging around in there for a while. The cheapest plants I ever got were sold bareroot by a local Wildlife Trust for a couple of pounds and established quickly.
Spalding bulbs are good for bulbs. Primrose plants say you save upto 30% by shopping with them and use well known suppliers. Lidl's raspberry canes are very good value, as are Thompson and morgan's raspberry canes when they are first advertised in late autumn: the price goes up in the spring. Poundland do very interesting offers and some are very good value for example peacock gladioli 10 bulbs for a pound. Ebay is always well worth a try if you know what you want. As always, the bargain counter of the garden centre, and offers on plants from the likes of Thompson and Morgan when they have just finished flowering are good.