You basically have two options here.
One is to go for a stock Dell PSU (either new or used) that will fit 100%.
If the stock PSU is too expensive or hard to find, you could put the whole setup in an other case and use a normal PSU.
The first is probably the quickest and easiest but may turn out to be costly.
The second may seem like a hassle, but may eventually turn out to be a better option. The reason being that from my experience with SFF PCs their main issue is HEAT! due to the lack of 'breathing' space they heat up and all sorts of bad things happen. Mostly burnt PSUs and blown capacitors!