The first thing to start with is that I am no where near an expert at this, however there are some key themes that I would sign post you to.
The first being brand consistency. Have a clear idea of what your brand is. When you are trading at various events you are potentially competing with many other vendors. As such you need to stand out and be memorable. The two things that many people reference is the name of your business and then a tag line. For example mine is The RGD Event List and the tag line is The place to find out about unique and different events across the UK. I also have an easy to identify logo which I use across everything.
When planning a brand you need to note down things such as colours and fonts you use. then when producing marketing materials you can reference.
Now to physical branding. This can be expensive and as such make sure you plan what you want and the design before purchasing.
I once again iterate I am not affiliated with the following but from personal experience have found the following to be really good.
Awesome Merchandise
For £5 including delivery you can get double sided business cards. This is great if starting out and don’t want to invest in lots of cards. Also if you have a theme but want to do subtle changes this means you can do so at an affordable rate. An example which I personally use, is where i take photos at different types of events. Thus I have sets of business cards with different photos but same logo and contact details.
https://www.awesomemerchandise.com/100-Awesome-Business-Cards-Deal
Vistaprint
This is a big company but the key part is they are forever doing offers. I have bought hats, T-Shirts, originally business cards, cups, etc. However each time I have managed to get between 10-40% the items. At one stage I got custom T-Shirts for about £4 a shirt. You can also sign up for their mailing list and they email the offers to you and give you access early sometimes. I would say though is check prices when offers are on to ensure they are good value. My next purchase I am planning is a flag for outdoor events.