There are many fine VA-accredited attorneys who can help w/ VA appeals. The National Organization of Veterans Advocates (NOVA) is a good website to find such an attorney. Best wishes w/ your case.
You should go to the find a lawyer section and make an appointment with a local attorney to talk about your case.
The information you obtain from this website is not legal advice.
As Drew mentioned there are a number of attorneys in your area that handle VA issues. Additionally, here is the link to the county VSO in your listed location who can provide free assistance in filing at http://www.cacvso.org/.
Death due to drug overdose in and of itself is not compensable (you would be filing a claim for Dependency and Indemnity (DIC) Compensation, however, if the drug addiction was related to a service connected mental health disorder and the accidental overdose was the cause of death, then you may be entitled to DIC benefits. If the death was the result of accidental overdose from prescribed medications for the drug dependency (specifically I'm thinking of methadone), then it is also worthwhile to mention 38 USC 1151 (compensation due to VA malpractice/negligence/lack of informed consent) if the prescribed medications were through the VA and your husband never was provided fully-informed written consent. . . this is unfortunately an issue I've encountered before.