Glad it turned out to be an absess and nothing something more serious, although you will need to keep an eye on Skrump as some hamsters are prone to absesses and either get them in the same place or different areas
The vet sounds like he did a thorough job to treat an absess, which is encouraging.
You get a few brands of probiotic in US - I haven't used any though so sorry I cannot personally recommend any. Check out the larger petshops, or online stores such as PetCareCentral, or FerretStore.