We describe the design, construction, and operation of an ion beam assisted deposition source for molecular beam epitaxy in ultrahigh vacuum. At a typical deposition rate of a monolayer per minute, the source may be operated in each of five modes: using self-ions from the vapor, self-ion plus noble gas ions from an additional gas inlet, both pulsed or continuous, or with complete suppression of ions. The source is based on electron bombardment heating of a metal rod or a crucible while the ions generated from the vapor are focused electrostatically onto the sample. Additional ions may be extracted from a noble gas stream injected into the ionization region. Examples for each of the different modes are given for Co deposition onto Cu(111), a system known to resist layer-by-layer growth.