Spacescape is a really cool program for generating space skyboxes. You can add layers for stars and nebulae and generate a 6-sided skybox for Unity. I made sure to colour each layer as red, green, or blue so that they could be split up properly in my shader.
half4 skybox_frag (v2f i, sampler2D smp)
half3 tint1 = tex2D (smp, i.texcoord).r * _Tint1.rgb;
half3 tint2 = tex2D (smp, i.texcoord).g * _Tint2.rgb;
half3 tint3 = tex2D (smp, i.texcoord).b * _Tint3.rgb;
result.rgb = (tint1 + tint2 + tint3);
result.a = 1;