I don’t have much money to spend, but I need some strong tyres and strong inner tubes for a 20 inch BMX wheel

your tire choice should be based on the type of riding you do.

most name brand tires are going to be equally strong but puncture resistance varies with the amount of threads within the casing.

I prefer Maxxis tires.

Tube selection is a simple matter. Buy them from a shop and you should be o.k. Stay away from no name discount tubes.

Also, the condition of your rims and rim strips, method of installation can have a significant impact on tire/tube life.