Horticultural quality clear acrylic sheet works well as greenhouse glazing. Acrylic is not as rigid as glass and therefore will never look exactly the same.
It may tend to dip slightly when used in the roof unless fitted into a timber frame with beading.
Aluminium framed greenhouses utilising the “W” shaped wire clips work better with a little silicone applied to prevent the clips from springing out which can happen in very high winds as the plastic flexes.
Most greenhouses accept 3mm thickness which is the same as the old horticultural glass. Some timber framed houses take 4mm. Some greenhouses have bent glazing panels at the eaves. These need to be cut in 2mm as 3mm will not bend sufficiently over that radius.
On timber structures with no rebate or slot the glazing is best secured with timber battens to spread the load on the fixings.
All panels including triangles and odd shapes can be supplied cut to size.
Horticultural acrylic is cheaper but may have imperfections.