How do I rite a letter to potential suppliers

Hi Kahn,
The best thing to do is research them first, you could do this online, word of mouth, local contacts etc.
In your letter, introduce yourself and your company. State your business needs, that you are looking for a supplier (either generally or for specific products) and that you have been recommended to contact them.
Keep it brief, suggest a meeting to discuss opportunities to work together, ask for a catalogue/price list.

If they don't contact you back, find another supplier.
If they do, have fun negotiating!

Good luck!
Moomin