You may browse our listing here for suitable HDB flats that suit your budget and requirements: https://www.propertyguru.com.sg/singapore-property-listing/hdb
Do note there are strict HDB rules and regulations that you must adhere to when renting a room. You must register your particulars with HDB. The minimum rental period for each tenant must be 6 months per application. You may refer to HDB's link here: https://www.hdb.gov.sg/cs/infoweb/residential/renting-out-a-flat-bedroom/renting-out-your-bedroom/regulations-for-renting-out-bedrooms
Hope this answers your question.