In the Uk some of the £1 shops (I expect there are some 1 euro shops on the mainland that this would apply too) that sometimes sell plastic plants but they are seasonal. Plastic roses in February (for cheapskate Valentines), other plastic plants for Mother's Day, I've seen some plastic "garlands" at Christmas, just green leaves like ivy, that work well.
For gnarled trees, like mangrove swamps, I found some plastic "bonsai" type trees decorating a jewellery store window. The shop had spares that they didn't want. Shop decorations are actually quite good if you get there when they are changing them over, the staff are usually happy to just get rid of the old ones (especially if they look dusty or damaged). You can sometimes get damaged pieces from florists too.
Car boot sales/fleamarkets sometimes have cheap toy trees that are worth aquiring to vandalise, some toy farm sets are quite good for foliage. (Britains used to have a very good range of trees, markeed as 1/32 scale but look like mature trees alongside 28mm figures, hard to find now). I have even used some Playmobil pieces occasionally.