I have had very good luck buying used Square D double throw transfer switches on ebay,
for reasonable $$$.
If you are not in a hurry, set up a saved search "square d double throw".
Here are some I found today:
eBay item number:295590601247
eBay item number:125607216207
And the one I bought from ebay for $200 a few years ago:

for reasonable $$$.
If you are not in a hurry, set up a saved search "square d double throw".
Here are some I found today:
eBay item number:295590601247
eBay item number:125607216207
And the one I bought from ebay for $200 a few years ago:
