This link will take you to an article about fertilizing Boston ferns: https://www.gardeningknowhow.com/houseplants/boston-fern/boston-fern-fertilizer.htm
The information is the same for Ostrich ferns as for Boston's, except that you won't need to fertilize after July 4. This way, new growth will have time to harden off before first frost. You won't need to fertilize again until new growth starts showing in the spring. You should be able to find 20-10-20 fertilizer at your local greenhouse that grow their own plants. Most commercial growers use 20-10-20 and would be willing to sell a couple pounds at bulk price. Take a sealable plastic tub with you. If you can't get it, just use 20-20-20.