I assume the moulds cost money.
For cheap and ultimately universal (make whatever you want) option make your own.
Some kiddy plasticine is easy as anything to roll, cut, sculpt into your basic brick or wall shapes. Texture with crumpled foil or sponge or whatever. Score brick marks and holes with tooth pick or pen. Set them on a board or old tray.
2 part RTV (sets with out heating) silicon can be pricy, especially in small quantities, but may still be cheaper than buying moulds, shop around. Flatten out or roll clay and cut to make edges to put on tray around your work. Mix and pour silicon.
A dirt cheap option is liquid latex. About 20-30 bucks a litre at an expensive shop, less for more. Simply grab a brush, massage in some detergent, then brush on latex. Massage in more detergent and wash brush once done. Let set over night. Give it at least 2 or 3 coats. You can also lay tissue between coats for extra durability and strength. Once done, pull off and trim edges with scissors.
These methods allow you to make whatever you want. You can also cast parts, adapt them, then remould and cast. The sky's the limit. A litre of latex will make heaps of moulds. Moulds of both materials should last year's, depending on numbers cast, but non industrial numbers should not hurt them much.
