I bought all of my 20 round 17s mags from SgtStryker, a member of this forum. He has been a great source of information and supplies thus far.
The link above states that the 10 round Pmags are NOT 17s compatible:
The Magpul 7.62x51 NATO (.308 Winchester) PMAG 10 LR/SR GEN M3 brings GEN M3 technology to the smallest possible magazine size. Perfect for hunting applications, depending on local hunting laws, and also for states where overall capacity is limited. Perfect for use with a bipod or when shooting off of a pack.
Optimized for use with 7.62x51 NATO (.308 Winchester) ammunition with an overall cartridge length less than 2.820", including M118LR
Additional commercial rifle compatibility includes the KAC SR25, DPMS LR308, LWRCI REPR, LaRue OBR, Bushmaster .308 ORC, LMT .308 MWS, Remington R-25, POF P-308, JP Enterprises LRP-07, Les Baer .308, Colt LE901-16S, the new Armalite AR-10A, S&W M&P10, and others
NOTE: NOT compatible with Armalite AR-10B, Noveske N6, Springfield Armory M1A, Rock River Arms LAR8, FN SCAR 17S, PTR-91, HK417 or any other rifles that utilize M14, FN FAL, or HK 91/G3 patterned magazines
True 10-round capacity that does not require limiters, pins, rivets, etc.
Next-generation impact and crush resistant polymer construction
Constant-curve internal geometry for smooth feeding
Anti-tilt, self-lubricating follower for increased reliability
High-quality stainless steel spring for corrosion resistance
Flared floorplate aids magazine handling and disassembly yet is slim enough for use with many pouches
Multi-use Impact/Dust Cover minimizes debris intrusion and protects cartridges from potential damage during storage and transit