Yes, exactly. You just need to find one with 2.25" diameter IN and 2.25" diameter going out for best applications retaining stock performance. If you go with total exhaust diameter larger than 2.5", there is no point and are losing performance. Do you really need that anyways? You can go with an exit diamter that is larger, but don't do something too extreme or "ricer-ish" with a 3" tip. I think it's ridiculous and defeats the whole persona of a "truck" with low-end torque and brute power.
Try to find something with a center/center or a same-side offset.
All muffler shops can take your exhaust, cut off the old muffler, and weld on a new one with new pipe. I dumped mine downward immediately following my muffler. I like it...
But yea...and you can definitely find used mufflers (a muffler is 97% good from personal sellers). Craigslist, eBay. You should be able to find a muffler for no more than $80 for one. If you are paying more, you are getting ripped off. If you can't find any, I can post links later. But yea...good luck.
Oh, and the single piping is fine and good enough for your truck. No need to send a Y-pipe or have more than 1 muffler...not necessary. If done right, a single can be jsut as loud on your truck and any dual cat-back exhaust.