You can fab your own pipes with a merge collector design. They don't need to be as fancy looking as those aftermarket ones. I don't think the exhaust gasses are that fussy.
I've built a few sets of pipes now. Here's how I do my merge collectors:
I made a jig that lets me quickly cut out triangular sections of exhaust tube. I sit 4 of these pices back to back and weld them into the space between the 4 pipes. A quick linish and it's all smooth and neat.
I use an off the shelf collector. It's just a piece of 3 1/2" tube that's been swaged out to a square shape on the end. This slips over the 4 pipes and then I knock the corners down to a snug fit on the 4 pipes. I then tack it in place. what takes a bit of time is panel beating the metal around the 4 pipes so it's a tight neat fit before welding. The better you can do this, the better the welds will look. My original ones I MIG welded but the new ones I've TIGed. I modified a large cold chisel by rounding off the blade end. This lets me work the metal down into the valleys between the pipes without tearing the metal or making too many marks. They can then be easily linished with a flap disc on an angle grinder for a clean tidy finish.
Here's what the merge loos like when it's all welded up.
Here's some anti-social ones I made
There are probably some other side pipe progress pics in my gallery.
Cheers