No particular products recs, but:

Schedule some extra time to swing by the closest sex shop before the session. If you're only using the item once you can go to the sketchy cheap sex shop instead of the fancy Babeland and grab a dildo for cheap. You'll probably still want to use a condom with it since cheap dildos are often made from non-bodysafe materials.

Alternately, if the Mistress has a mailing address she's willing to share you can purchase a dildo online and have it sent to her ahead of time. The same caution about materials applies.