Extinction and reddening dataset Where can I find a dataset containing the interstellar reddening and extinction values for all globular clusters, preferably in the HST/WFC3 system.
Note that they are related via the reddening law, i.e.
\begin{equation*}R_\lambda = \frac{A_\lambda}{E(B-V)}\end{equation*}
where $A_\lambda$ is the extinction at wavelenght $\lambda$, and $E(B-V)$ is the  color excess for $B-V$. In literature is commonly used $E(B-V)$, while in practice is usually needed $A_\lambda$, to switch back and forth from apparent magnitude $m_\lambda$ to absolute magnitude $M_\lambda$ with
\begin{equation*}m_\lambda=M_\lambda+A_\lambda+(m-M)_0\end{equation*}
using the absolute distance modulus $(m-M)_0$.

Back to your question, classical references for the color excess of galactic globular clusters are the Harris catalog and the Schlegel maps (I would prefer the second). A nice solution is the Python module dustmaps (Green 2018); in the documentation you find all the available maps (here). 

Finding $R_\lambda$ is more tricky. You can search in the literature for tabulated values (e.g. Tables 14 and 15 in Sirianni et al. 2005), otherwise a rough-but-still-working approach is to take a $R_\lambda$ vs. $\lambda$ plot (e.g. the one in Fitzpatrick 1999) and to extrapolate by-hand $R_\lambda$ at the effective wavelength of your filter. Note that usually are presented several reddening laws, distinguished by $R_V$. This is because they're not universal and depends on the environment; a reasonable choice for GGCs is $R_V=3.1$ .