You can find inexpensive fish tanks at garage sales and thrift stores. You can also look in the classified section of your local newspaper to find affordable aquariums. Most of the tanks sold in these types of places are used, so make sure they have no cracks and do not leak. You need to rinse the tank thoroughly with clean, dechlorinated water before adding your fish to a used fish tank.
Promotional offers are another way to obtain an inexpensive aquarium. Pet stores often offer aquariums at inexpensive prices during grand openings or other sales events. This is particularly true of small aquariums, with a capacity of 5 to 20 gallons, but sometimes you can find larger models as well. These types of sales are not limited to brick-and-mortar stores; online retailers often offer similar sales. The only drawback to this approach is that you have to pay for the shipping costs. Many companies offer discount vouchers for online or in-store purchases.
Educational and non-profit institutions that keep live animals often give away old aquariums or sell them for a nominal fee. You are unlikely to see these types of opportunities advertised, so you have to reach out to these institutions yourself.