I looked all over the net before I bought - Best deal I found for my 870P was from Advanced Specialty Products Inc.
Here's a link ASPI Speedfeeds Hope this helps.
Botach has them as well. Be careful when dealing with Botach, call first to make sure your item is in stock otherwise you may be in for a very long wait! http://www.botachtactical.com
HTH,
Justin
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Justin T. Huang, Esq.
late of Kennett Square, Pennsylvania
Yes be careful of Botach. Prices seem good but they may be out of stock on it. Many people including myself have had problems with them with shipping, returns/exchanges etc.