Brenneke makes slugs at a very resonable price. They fly extremely well for a slug, I used them in my smooth bore 20 guage when nothing else would hold a consitant group. Check them out, http://www.brennekeusa.com/
They make both sabots and rifled slugs, either will work in a rifled barrel. Their slugs are unique in that they have a stablizing wad/jacket that fits around the slug.
They are not made for long range shooting, they are made for accuracy and consitancy. It will tell you right on the website what the maximum effective range of each load is. This is based on kenetic energy. Some rounds my surprise you, as they are only recommended for yardages up to 60 yards, others 80+ But like I said, these are made to be accurate and consistant rounds, not built for speed and long range shooting.
Another option... you can use regular rifled slugs in a rifled barrel. You will still experience increased accuracy and performance with rifled slugs in a rifled barrel, just not as much as if the slugs were sabots. If money is really an issue for you, than you may consider this, as rifled slugs are about as cheap of ammunition as your going to get.
Sabots are going to run you atleast $10 for a box of 5 (from what I have seen). The sabots I use are $17 dollars a box. Yes, expensive... but not when you only have to shoot at a deer once and you know it's down. If I can harvest 3 deer with one box of shells (2 shells for the target range, and 1 for each deer) than the 17 dollars doesn't sound too bad.