1 1/8" slugs are available, but are typically used for people with smaller thumb sizes. The standard slug is (and most commonly used) is 1 1/4". If you pick up one of those, you should be in business.
1 3/8" slugs are available, but I've only seen those used to cover up mistakes.. And the newer interchangable thumbs are (I believe) 1 1/2".
S^2