Tromix Lead Delivery Systems is a big name (possibly the biggest name) in the Saiga community, for good reason. The Inola, OK based company (that's a little town just up the road from me, by the way) is so busy, in fact, that they're not taking new orders for at least two years.
That's pretty busy.
They still find time for some pretty awesome customer service though. Check out this power chair Tromix founder Tony Rumore is building for a customer who can no longer walk but wants to continue hunting.
If you need some Saiga shotgun customization (or if you feel the need to buy one) then you should definitely check out Tromix. Just be patient.