This sort of letter can be extremely useful, and save you a lot of time and effort, but you have to be specific about what you're after.
A few basic rules:
Ask for information in clearly defined areas, to prevent them sending you a library about their other products.
You are not committing yourself to anything at this point.
Only when accepting an offer or making a formal offer to purchase are you in any way obligated to the other party.
You can refer to things like price ranges, discounts, and other possible benefits for yourself as a customer.
The letter can be quite brief, but make sure you're asking for what you want. It's often an idea to quote things like their product references, so you're sure they understand what you're looking at.
Request for product information / trade account information
We're currently looking for a supplier for our (insert definition of what you need). Your advertisement / website looks like you have what we want.
Please forward information regarding your products and services. It would be appreciated if you also provide a comprehensive list of your charges, and a contact to discuss business details.
Optional paragraph where additional information is required or relevant
Please also provide information regarding:
It will be appreciated if this information is provided for each class / brand of products available, to give us a basis for comparison.