You have a wide choice of places to buy a tractor for lawn mowing.
Some offer low prices, some a wide selection, some high quality, some excellent service, some convenient evening and weekend hours, and some offer knowledgeable sales staff. You may find several of these characteristics at one store, but you will seldom find all at any one place.
Discount stores often sell one or more brands of lawn, lawn-and-garden and garden tractors. The tractors may carry the store brand or may carry the manufacturer’s brand name. The tractors tend to be low-cost and of lower-quality than you will find at a dedicated dealer. In many cases, parts and service are not available. You will probably have to deal with the typical discount store employee, so don’t expect much helpful advice. Delivery may or may not be available. Quality of predelivery service is questionable. The advantage here is price and convenience (of the original sale).
Mass merchandisers often sell tractors with a house brand name, but different models may come from different manufacturers. Manufacturers can change from year to year, so parts availability is problematic. Salespeople may be somewhat more knowledgeable than at discount stores. The tractors are typically equivalent in quality to discount-store tractors, but the sales staff may be more knowledgeable, and parts and service may be available.
Dedicated lawn and garden equipment dealers tend to have higher-quality tractors, although a given dealership may carry both inexpensive and high-quality lines to appeal to a broad range of customers. Some carry top-of-the-line equipment from ag machinery companies. Quality of sales and service staff, and parts availability can be very good. You should be aware that some of these dedicated dealers cater primarily to grounds maintenance professionals, and they will schedule service of commercial equipment ahead of homeowner equipment rather than on a first-come, first-served basis.
Farm equipment dealers generally offer high-quality equipment, knowledgeable salespeople, good service and good parts availability -- for a higher price. Some ag equipment companies have separate lawn-and-garden dealerships in addition to their ag dealerships. Some also offer more than one line of equipment. These second-tier lines are designed to compete with mass merchandisers while still offering the sales, service and parts advantages of a dedicated equipment dealer.
Once again, you have to decide what is important to you and what you are willing to pay. You will save money in the short run at discount stores and mass merchandisers; you will pay more but get better-quality equipment and service with dedicated lawn-and-garden dealers or farm equipment dealers.