Know the Types
There are several kinds of vending machines and you should be aware of them. This will help you in knowing whether you have all the types that you are interested in or not. It will also give you information about where these can be found. It is best that you are at home with this information before you start collecting.
Vending machines have a life and they eventually reach a stage where they are phased out. You can ask the vendors that where do these machines go at the end of their natural life cycle. You can also check with them if they have any that are not of use and are available with them. They will be more than happy to move any such machines as they are not of any use to them and they only take up space.
See online if any such machines are up for sale. You will need something that is old and does not cost a fortune unless you are looking forward to spending as much as it takes. You can find some on auction sites such as ebay and amazon.com.
You can also look at forums in the related field and check with posters if they have one to sell or are aware of someone who might be interested in selling one.
Junkyards are likely going to have a ton of these. You can go and check with the staff there as well as go through it yourself to find any that are available. Obviously you will need to clean them and get them in proper order but this will be the cheapest way to find the most variety as well as pay the least.