It's your son's wedding.  You deserve to and should look your best.  However, you should hold off on that shopping trip until you have read the following tips.

Traditionally, the mother of the groom purchases her dress after the mother of the bride has purchased her own dress.  Her choice of dress is meant to subtly dictate the type of dress the mother of the groom should purchase.  This may seem like a bit of an old fashioned way to go, but the suggestion is to at least talk with the mother of the bride to get a feel for the type of dress (the length, color, etc.) she will be wearing. Once you have this information, it will be much easier to choose your own dress and feel confident that your outfit will coordinate with the entire bridal party.

As a general rule, the mother of the bride is the one to initiate the conversation about wedding day attire.  Usually she will call the mother of the groom after she purchases her dress.  This is to inform the mother of the groom about style, length, color, etc. Presumably the reason for this is to prevent overdressing or choosing colors that will clash with the rest of the bridal party.  If the mother of the bride does not initiate this conversation, the groom should subtly suggest to his bride that her mother might was to give his mother a call concerning wedding attire.  The brides choice for her own wedding dress and that of the bridesmaids dresses are meant to subtly dictate what the mothers will wear.

You should probably start shopping for your dress approximately 6 months prior to the wedding day.  This way you will have plenty of time to choose the perfect dress and schedule fittings for alternations if needed.  

When choosing your dress there are a few things to consider.  If the wedding is in the evening, a formal dress is probably the best bet.  If that is really not your style, you might consider a dressy suit.  As a general rule, neutrals colors are always safe to wear. Most mothers of the bride and mothers of the grooms should avoid white (the bride's color), black (mourning) and red (flashy).  However, if you have your heart set on one of the "taboo" colors, talk with the bride.  If she really doesn't have a preference and the color you choose does not clash with the mother of the bride or the bridal party, then go ahead and purchase what you like. However, if she seems reserved about colors, you should probably stick to the neutral colors rather than making a fashion faux pas with the bride.  

While the bride does not have all the say in your choice of dress, you should really try your best to respect her wishes.  As a general rule, even the bride doesn't say anything directly about what you should wear (she may be too reserved to state her wishes), it is always a good idea to take your cues from the style, length and colors of the bridal party's attire.  

You don't need to look frumpy and matronly at your son's wedding. However, if you are questioning a style you would like to choose, it may be best to consult with the bride before actually making the purchase.  If she seems concerned (maybe a bit too sexy), you may want to choose a style that is just a bit more subdued (not frumpy or matronly).