Where can i purchase a weber spirit conversion kit?

Contact weber to see if they have a conversion kit. The venturi for propane and natural gas are different sizes due to the different burning properties of the gasses.

you can change them but you need to change the jets there are conversion kits for the weber cooking systems.

Weber stopped selling conversion kits due to some legal action in 2008. There are no longer kits available. What you would have to do to covert it is order a replacement gas valve for the NG version of your grill just to get the orifices. Then you have to replace the LP regulator with a NG regulator. Typically most NG grills do NOT have a regulator but local building codes will require a NG regulator and a different gas line hose.
Conversion of a BBQ gas valve to NG is a matter of drilling out the orifice to a larger hole in an LP orifice.
You will not find anyone that will perform this conversion for you due to legal liabilites.
If you want to convert it... you will either have to order the whole gas control valve for NG or drill out the orifices yourself in your current gas valve hoping your got the orifice hole size correct when you drilled it. Mind you all liability for doing this lies solely with you.

This site has all weber parts:
http://www.clagrills.com/allgrillparts/weber/weber-models.htm

http://www.clagrills.com/gasgrillparts/weber/weber-gas-grill-parts-home.htm

Thanks for choosing FixYa,
Kelly

Unfortunately, there's nothing you can do. According to Weber, there are no conversion kits, as your grill is not convertible to propane. Like wise, a Weber propane grill is not convertible to natural gas.

Link to your Owners Manual (see page 2 - Warnings):

http://help.weber.com/manuals/pdf_files_2011/Gas/Spirit%20ES%20Series%202007newer/OwnersGuides/Spirit_210310_NG_OwnersGuide_89961_022811.pdf

Weber sells conversion kits. Basically, you need a new regulator and manifold to accommodate LP instead of the existing regulator/manifold setup for natural gas. Unfortunately, Weber does not sell their parts online. Their parts phone # is: 877-731-8081

Sorry, but there are no conversion kits available for Weber Grills. They were no designed, tested or approved to be converted from one gas type to another.

For confirmation of the this, see FAQ link to Weber, below.

http://help.weber.com/faqs.aspx

Apparently, Weber does not make a conversion kit to convert the grill from propane or from natural gas to propane. According to the Owners Manual - "Liquid propane gas is not natural gas. The conversion or attempted use of natural gas in a liquid propane unit or liquid propane
gas in a natural gas unit is dangerous and will void your warranty." (From Page # 3 of the Owners Manual)

See below link.

http://www.weber.com/help/manuals/pdf_files/2008_updated/89525_030308_2008.pdf

can not help, you are in the wrong forum.

Try replacing the regulator.