Hi, any garden centre and larger supermarkets sell Geraniums. There are wild Geraniums in the UK that you could pick for free, mainly in woodland margins near fields and moorland.

Do you mean hardy geraniums or pelargoniums. Often people use the name geraniums for pelargoniums . Garden centres will have them now.

Any variegated plant will do except for the humble spider plant. It is a lovely variegated plant but makes and stores sucrose. so a negative starch test.

Ivy is excellent as it has a very waxy cuticle and helps explain the need to dip in hot water before going into the alcohol for decolourisation.

There are several trailing plants that will work out cheaper as there are lots of leaves/plant.

I do notice that the white zonal geraniums have almost no zoning whilst the coloured ones are a pronounced colour ( useful as I wanted the white in a different place!)