In my new playthrough, I want to get the Indie courier's duster to go with my current playthrough. Since I read that to get the Indie duster you have to have equal or lower rep with all the three factions so I decided that I wouldn't do any of the NCR, Legion, or Strip quests until I complete all of the DLCs. Here is where I'm stuck usually I would do the NCR quests in order to raise my level, but since I can't I had to go outside of the main factions and do some of other quests that don't relate to the three main factions. I'm already at level 13, but I've ran out of ideas to get that extra two levels so I can do Old World Blues. I would appreciate if you guys can offer up some ideas on how to get those two levels.

I don't think you get Accepted with NCR until like you do Forlorn Hope quests.
So any quests prior to Forlorn Hope (Mojave Outpost, Primm, Novac) should be fine.
Other than that I suppose you could sprint on up to Vegas and take on Kings, Followers, Van Graffs or Strip Families quests.
Or go to Jacobstown, or Great Khans.
Oh and kill stuff. Lots.

But lvl 15 is really easy to get to, just kill some stuff, go south to Searchlight and kill ghouls and bighorners.

Rather easy actually. Stick to killing wild life, Big Horners are your friend for a quick few levels sense they have quite a few packs around the map.

You could also fight at the Thorn, it'll be costly due to ammunition/meds but the xp can be quite useful.

Thanks I was mostly thinking of using the quests near New Vegas that I forgot about Camp Searchlight's ghouls and killing wildlife.

Since you're going for independent, how about killing Legion`? That way tons of quests are still left on NCR side for when you get back from The Divide and you're gonna end up killing Legion anyway so why not give it a head start?

Never even thought of that and since I get my rep back up with the Legion after I kill Benny I guess I could go on Legion killing spree. Though I wonder can I skip meeting House and just go straight to Benny?

Yes.

Callville Bay, Ruby Hill mine,Black mountain,New Vegas ruins,North of Goodsprings, and the area surrounding nipton reststop are all good XP spots.

Just wondering, is there a way to get my reputation with the strip down without killing people on the strip? I'd rather avoid pissing off my own securitrons (it's just for role-playing purposes, I can handle a few securitrons but I don't think it would make much sense for me to kill my own city's law enforcement)

Have you done Honest Hearts yet? You can easily handle that at level 13 and it'll more than get you up to where you need to be.

I haven't, I've been focusing on doing the DLC's based on the levels recommended. I was going to do OWB first then DM next followed by HH and then finally LR. I'll try HH first and see how it goes.

Just do what you want but before you go to Canyon Wreckage kill some faction people until they are all below neutral then enter the DLC, sorted.