Here's a stamp kit of numbers to go along with the Stamp Act Alphabet Rubber Stamps, or to use by themselves. They are about 5/8
Stamp Act Numbers Rubber Stamp Kit
Make Your Own Stamp Large Self Inking Stamp
Identification Rubber Stamp No. 16 - Stampmore
The Ultimate Guide to Deposit Only Stamps
Trodat 5756/M Numbering Machine
Rubber Stamp Month Numbers, Vertical for 0.5 Grid, Perpetual
Cavallini & Co. Rubber Stamp Set - Numbers & Symbols
Custom Stamps, Business Stamps
Cavallini & Co. Rubber Stamp Set - Numbers & Symbols
Dialable Number Stamp