There are companies online that will send you inserts and they will make them off of that. The best thing to do would be to call some audiologists (ear doctors, they do hearing aids as well). They should be able to either fit you with a pair or get a pair made for you. They won't be cheap, expect to pay at least a hundred bux if not 200. You can also get "blackout" plugs that will only cut out certain frequencies. They work rather well.
What are you trying to do? There are different varieties of ear plugs, from high isolation models to those made for musician's and sound techs that have a broad, even loss over frequency. Etymotic Research (http://www.etymotic.com/default.aspx) and Westone (http://www.westone.com/) are two of the more poular ones. There are many other providers and a number of them actually use the ER inserts, but may have more convenient locations for fitting.
The Etymotic Research ER-15 models are very popular with musician's and sound techs because of their flat attenuation over a broad frequency range, but if you are wanting more isolation rather than a flat spectrum there are other models that may be better choices.
The manufacturers' sites provide information on local audiologists that can do the custom molds, some of the manufacturers also offer this service at some of the audio trade shows (NAMM, NSCA, etc.).
I can vouch for the awesomeness of the ER-15s. I got them last year, and while they aren't as comfortable as foam plugs for extended wearing, they sound excellent. It's possible to understand conversation from some distance, unlike foam.
If you go that route, go to an audiologist. The mold making process would be difficult to perform on yourself, to say the least. Probably good to leave the en-gooening to the professionals. After the molds are made you'll have to wait a couple of weeks, and then go back to the audiologist to make sure that they fit, and if the audiologist is cool, a full hearing test to make sure that the response is flat.
The whole thing cost me $150, but after having lots of fun with my ears as a kid, I figured that I should at least try to treat them decently when at work. They're also quite useful for driving around with the windows down. Conversation is easy, but much less unpleasant wind in ears (the feeling of continuous popping tends to make me very cranky).
I personally use ER 25's. I got them from Precision Laboratories in Florida. Their website is precisionweb.com. I paid $100 but I got a group deal on them. I really like them and are really comfortable and dont hurt my ears at all which is to be expected given they are custom. ha ha.
The cheapest way to get them is to talk to musicians or a monitor engineer. Normally they are sponsored or get deals on them.