If salmon belly sashimi is too expensive, cooked salmon belly is a good alternative - the former usually costs more than S$100 / kg while the latter costs S$15.90 / kg at NTUC Fairprice. Compared to salmon steak, salmon belly is rich in good fats, which is exactly the nutritional purpose of eating oily fish. After tasting baked belly, I have never bought any other cut of salmon.
The way I like mine done is baked, with freshly ground black pepper and salt, either garlic granules or freshly diced garlic, and a teaspoon of sambal chilli on the side. If you want crispier skin and flesh, you can fry it, but be careful with the flame because you do not want to hit smoke point when the Omega-3s get oxidised. Additionally, do note that a larger amount of oil is usually released when the belly is fried as compared to baked, and it may be a little rich / nauseating to drink the oil out of the pan.
Raw salmon belly (non-sashimi cut) is usually available at NTUC Fairprice Finest, Bukit Timah and Thomson Plaza branches, at S$15.90 per kg.