[RockMybes]

N have a bunch of old cars and new cars how do I get correct tire size , tried to search but to many things came up

[GT40Racer]

Where are you located?

These are some options if you are in US - 
some original tyres are available as spare parts, 
slot.it/NSR/Sideways etc make replacement rubber tyres
a few places make urethane tyres - eg Paul Gage, Supertyres (Yellow Dogs) MJK - Australia and WASP UK are some companies that make them
Silicone Tyres - I don't use them but Quick Slicks and Supertyres make silicones

In most cases retailers will have tire selection guides. For example on Paul Gage's eBay site you can search for tires by brand and car.
Some also have the measurements of the tire so you can measure the hub and work out which tyre will fit.

As for whether you go urethane or rubber or silicone - well that is a whole other conversation - but ultimately comes down to what people run if you race with others or it might be a case of trying a few options and see which one you prefer.

What type of track are you running on?

[Johnno]

N have a bunch of old cars and new cars how do I get correct tire size , tried to search but to many things came up

You need to measure the wheels, I've attached an image of the wheel areas that you need to measure so grab those vernier calipers and wheels. 

You will find it easier to create a spreadsheet with cars and wheel fields then enter the measurements directly into the sheets.