Elk, if you PM me your email address, i can send you a PDF of a diagram of Shimano Tiagra brifters, with all the parts shown in detail and listed by number, including the various shims available for them. You'd want the Left and right hand 8 degree "adjustment blocks". I bet you can get any LBS that is a Shimano dealer to order the shims for you by the parts #, and then install them on your bike for not very much money.
Tiagra brifters are a different shape than the R600 brifters. The PDF also has a column indicating the Tiagra shims are not interchangeable with Shimano ST5-600 or ST6-600 brifters.