I concur with LINEMAN that Agri Supply is the best source for Disc Harrow parts in the Southeast USA.
Your tractor brand and model should be part of your T-B-N profile, as should your location. Tractor model tells us tractor weight, width and wheel size. Freight is a large part of total Disc parts cost.
A Box Frame, Tandem Disc Harrow with 18" diameter pans is the lightest Disc Harrow which is effective at Secondary Tillage. Meaning a Disc Harrow with 18" pans will smooth previously plowed land with several passes over the field. Early passes are a rough ride. With each additional pass ground is softer and tractor/disc unit is more likely to bog down.
Disc Harrows with 18" diameter pans have around 35 pounds implement weight bearing on each pan. It takes around 50 pounds implement weight bearing on each pan to be effective, meaning reducing furrows in one or two passes over the field. Generally, this is Discs with 20" diameter pans and larger.
The weight of your tractor, not horsepower, is the primary consideration on whether you can draw a ground contact implement. How aggressively Disc gang angles are set by operator partially determines Disc Harrow draft force resisting tractor.
A Disc Harrow with 18" diameter pans is useless for Primary Tillage, in my opinion. Some, however, may be willing to expend fuel and time to cut a small area six to ten times in different directions to get top four to five inches cultivated. Not I.
With each additional pass ground is softer and tractor/disc unit is more likely to bog down.
Consider a PTO powered roto-tiller. Roto-tillers and Disc Harrows are both soil mixing and field leveling implements. Suitable width Roto-tillers perform well behind tractors with as little as 1,700 pounds tractor weight and 20-horsepower