The FA or Football Foundation only offers funding for the replacement of unsafe goalposts and not for purchasing new ones. Therefore, if you are replacing unsafe goalposts you would be eligible to apply for goalposts under the Football Foundation Small Grants Scheme.
You can find out more about the replacement goalpost grant at the following link: http://www.footballfoundation.org.uk/funding-schemes/premier-league-the-fa-facilities-fund-small-grants-scheme/application/replacement-of-existing-unsafe-goalposts/
The application is via Grantshot: https://grantshot.org.uk/
If you are purchasing new goalposts rather than replacing unsafe goalposts we normally advise clubs to approach their local parish councils or local authority, to see if there are any local funding pots that can be accessed for this type of purchase. Living Sport are the local body representing Sport England and have some good funding advice on their website: http://www.livingsport.co.uk/funding/