I have a budding Ork army but I don't have any spare Lootas. The best thing I found to do was just make my own. I have picked up two boxes of the Loota/Burna boys and made about half a dozen extra lootas out of regular Ork Boys. It is crazy easy. Just find any medium to large gun, glue it on the boy, and then add something else small to it. Ive got one guy armed up with a lascannon with part of a IG vox caster and a flamer tank on it.
Something else you do is just throw a regular boy with big shoota in there. Put him in the middle of the unit and he fits right in. If anyone asks (which I haven't had anyone do) just tell em he hasn't had enough kills to get his kit built up yet.