In keeping with several other replies to Pierre's topic here, I group I work with uses the 5351 in several of our projects but we don't use them in near the quantity that Hans @ QRP Labs does.

When they started to become scarce, I bought a few of the last ones available from Mouser, and when that batch got close to being used up, I went looking to see if there was another source.  I don't remember but I think Arv Evans over on the BITX20 reflector mentioned that the Chinese made an equivalent device called the MS5351M.  In searching for those to see if they were suitable, I came across a website called "Chipmall" (www.chipmall.com).  I checked them out and they actually had the MS5351M in stock, showing a quantity in excess of 5000.  Turns out I had to register as a customer before I could order, but after doing that, I took a chance and ordered 30 of them.  The price was less than $1.00 USD, but their only shipping option was DHL at $30 which was pretty expensive but after doing the math, $0.77 USD each in 30 quantity - $23.10 + $30 = $53.10 total cost for 30 of them delivered.  Now, divide $53.10 by 30 and you get $1.77 each which is not really that much more expensive than paying $1.09 each + $7.50 shipping from mouser ($40.20 total for 30 and if you do the math on that, they come to $1.34 each which is only 0.43 difference.)  That was well within range of keeping our close to break even point on target with no change in actual prices of the kit, we just dropped free postage and kept the kit price the same but added shipping & handling which actually saved a bit of money on our end as USPS has raised prices on shipping several times lately.

Now, once I got the initial order of 30 in, using one of our kits as the base, and finding an actual test socket for that SOT-10 format chip on Amazon, I designed and built a test set for the 5351.  Testing the MS5351M's against the Si5351a using the exact same parameters for both, and scope checking all 3 clocks, there turned out to be no significant difference between the two.  Even the I2C address of 0x60 was the same. 

I'm posting this here so that anyone needing reasonable quantities of the 5351 chips can, if they like, obtain them at prices that aren't terribly elevated from what the Si5351a-b-gt was before the shortage.

Jim, W0EB