Try Mike at Crown City Pawn Shop in Pasadena, (626) 793-5404. Their transfer fee is $20.00; I've done a handful of transactions with them, both selling and buying, and Mike is a good guy. No gun shop attitude there at all.
By the way, the practice of charging sales tax on a private party transfer based on the price of the gun appears to be illegal:
"SALES AND USE TAX ANNOTATIONS
California Board Of Equalization
295.1675.600 Service Charge for Sale of Firearms. Since the change in the law requiring private parties to sell firearms only through licensed dealers, a company asks whether the fee it charges to file the required state paperwork to the Department of Justice is subject to tax.
Whether the company is liable for sales tax on the transaction depends upon
whether the company is the retailer of the firearms. If an owner of a firearm and a purchaser have negotiated the terms of the sale in advance and then bring the firearm to the company to meet the statutory requirements of the Penal Code, the company is not the retailer of the gun and does not owe tax on the fee or any other charges for the firearm. If, however, an owner of a firearm brings the gun to the company and requests the company to find a buyer, the company is the consignee and the retailer of the firearm, and is liable for sales tax on the sale. In that case, the company must include all fees and charges in the measure of tax. 1/9/92;
2/20/92. (Am. 99–2).
(Note: On and after January 1, 1999, the Department of Justice fee is not
includible in the measure of tax, but all other charges remain subject to tax.)"