As a buyer, what are some good methods to obtain a price quotation for raw materials from a supplier without letting my regular supplier know?

Why to hide at all.

Our regular supplier should always be intimated that we are professionals and they can not take our relationship for granted. We can always free to check the market and get the raw material prices from the suppliers. Old relationship is good thing but loosing margin because of that is not great to pursue, and our supplier should abide by this.

As per my understanding and experience they do appreciate this too.

There are a couple of solutions:

1- going with the STD process of RFQ. but in this case the supplier knows you are looking for it and, at the end, if you RFQ regularly without releasing a PO, you will get the result that the supplier knows you are cheating and just asking and would not be willing to answer  you anymore. 

2- promote an e-auction access only to qualified suppliers. in this way you can benchmark the current price, at the same time, from a certain number of suppliers and chose to which release the future PO. Normally you can tender an e-auction every quarter without having the supplier suspicious on the scope, cause infact the scope is to release a PO. This method works quite fine and, more, is extremely fast.

Direct contact with supplier to get quotes with other company reference 

1. Use the portal of the vendor web site to send RFQ.
2. Direct contact to the new supplier via email or fa

To have a rate repair quote from the existing and new supplier .

My best tip for negotiating with a supplier is to always get the service quoted by at least three other suppliers. I let the supplier know that I am getting quotes and will go with the best one available. This creates some competitive pricing. 

without letting your regular supplier know ?

what difference does it make ?

In my opinion you should let him know ..for competency & transparency

It is not only your right to do so but also your duty

You / your company must have a standard policy /Guidelines one of them is (at least 3 Quotations from 3 different suppliers (as mentioned in some answers)   

Floating Annual RFP is one important factor to get the price and then E bidding is the most popular and transparent way of getting genuine prices and without knowing the other suppliers

With the technology available today, it easier to analysis the market cost by enquiring/publishing the requirement as mystery buyer and get the quotation for our analysis to drive the correct cost.

- Regularly we obtain 3 quotes for goods / service from 3 different suppliers or service provider  , but in case of emergency we can obtain 1 quote  

- Arranging for yearly tender or contract will facilitate process of selection

The below methods can be adapted for price inquiries.

1. Floating direct RFQ or through concealed identity, if required.

2. Direct calling on vendors followed by email communications.