A good source of spring steel in a useful range of sizes is a pack of steel shim stock, this can be purchased from a good Engineering suppliers. Alternatively for small amounts I have always found that a standard Automobile set of engine feeler gauges have many useful sizes. Both items are reasonably priced because they are used in quantity and therefore low cost to purchase, the moment the word gun comes into things the price is elevated beyond the norm.

The only lessons in my life I truly did learn from where the ones I paid for!