You can also cast your own.
Get yourself a casting kit with RTV (room-temp vulcanizing) silicone rubber and resin in. If you were in Nolfolk, VA rather than Norfolk, UK I would recommend an Alumilite starter kit from Hobby Lobby. But, you can likely find something similar.
Make up five or six bases with the texture you want - use whatever plastic bases you want and model the tops. You could do that using the textured plasticard, or you could use Green Stuff etc. Just make sure they are good and solid and everything is sealed.
Glue those to a piece of smooth plasticard with plastic cement (so they are good and sealed to it).
Make a box around it (lego bricks are good) and seal the edges with oil-based clay (plasticine).
Mix and pour the silicone according to the instructions. Wait a day and demold.
Mix the resin according to instructions and pour. These are simple, single-sided molds so they should be easy. Winkle out any air bubbles with a pin.
Repeat the last step as many times as you need.
If you need different size bases, you can make as many masters as you need.