Pretty much any flashgun with a standard hotshoe. The 3800N is extremely basic and does not take advantage of any dedicated flash functions, so you'll find loads of cheap or even free basic manual or auto-exposure flashguns on auction websites and also on FreeCycle.
I'd suggest a basic manual flashgun as you then fully control it. Automatic flashguns try to second guess what it is you're photographing so can often over-expose the subject. You also learn more using a basic manual flash which is probably the only reason to buy a 3800N in the first place.
Answers & Comments
Pretty much any flashgun with a standard hotshoe. The 3800N is extremely basic and does not take advantage of any dedicated flash functions, so you'll find loads of cheap or even free basic manual or auto-exposure flashguns on auction websites and also on FreeCycle.
I'd suggest a basic manual flashgun as you then fully control it. Automatic flashguns try to second guess what it is you're photographing so can often over-expose the subject. You also learn more using a basic manual flash which is probably the only reason to buy a 3800N in the first place.