Purchasing Parts for Trek T30i

[jbswagger]

Hi,

I dug a bike out of the garage that I have had for a while but not used for a few years and, took it a local bike kitchen which give a basic check up and told me some parts that I need. Having always bought stuff like brake pads inner tubes and tyres from Halfords I've never purchased any cycle related parts from an online supplier. I need a chainguard and some other bits. Will the supplier automatically know the size of the chainguard that is fitted to a Trek T30 or do they come in different sizes.

Any recommendations for any parts suppliers welcome.

Thanks for your help