If the greenhouse was originally glazed with horticultural glass, but you have problems with vandalism (also some allotments disallow glass greenhouses - check rules) there are options:
1. Order toughened glass made to measure. Very expensive, but permanent and good light transmission.
2. DIY plastic glazing. PVC is cheapest. Acrylic (trade names Perspex, Plexiglass etc) is more expensive, longer lasting and slightly better light transmission. You can get this mail order or at a local diy shed. Can be cut to size easily with a fine-tooth panel saw.
3. Twin-wall polycarbonate. Stronger, warmer, poorer light transmission. Probably won't be compatible with the glazing clip system on the greenhouse, as it is thicker than glass, so you will need to improvise a fixing system. The gap between the layers can harbour pests and green algae.
4. Salvaged Glass. Clad the greenhouse with assorted patio doors, velux windows etc from skips then fill in any gaps with ply or whatever. Often double glazed, so great insulation and always toughened. Looks a mess, or not depending on your approach.
5. Short-term bodge. Tack or tape on some polythene sheet. Needs care to avoid sharp edges damaging the plastic, and won't last more than a couple of years.
6. Cover with netting, and call it a fruit cage.
7. Cover with plywood and call it a shed. :)
