Using 15-amp thermostat on a 20 amp circuit
First off, regarding fixed electric space heating equipment the National Electrical Code has this to say:

424.3 Branch Circuits.
(A) Branch-Circuit Requirements.  Individual branch circuits shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which they are rated.
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(B) Branch-Circuit Sizing.  Fixed electric space-heating equipment and motors shall be considered continuous load.

Since the heater is considered a continuous load then the branch circuit serving it has to be sized 125% of the load. Consequently, a 20 amp circuit can only serve a 16 amp load.
The Code also says:

424.19(C) Unit Switch(es) as Disconnecting Means.  A unit switch(es) with a marked “off” position that is part of a fixed heater and disconnects all ungrounded conductors shall be permitted as the disconnecting means required by this article where other means for disconnection are provided in the types of occupancies in 424.19(C)(1) through (C)(4).
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(3) One-Family Dwellings.  In one-family dwellings, the service disconnecting means shall be permitted to be the other disconnecting means.

So, an additional disconnect is required for the heater besides the main service disconnect.
Then the Code has this to say about thermostats:

424.20 Thermostatically Controlled Switching Devices.
(A) Serving as Both Controllers and Disconnecting Means. Thermostatically controlled switching devices and combination thermostats and manually controlled switches shall be permitted to serve as both controllers and disconnecting means, provided they meet all of the following conditions:
(1) Provided with a marked “off” position
(2) Directly open all ungrounded conductors when manually placed in the “off” position
(3) Designed so that the circuit cannot be energized automatically after the device has been manually placed in the “off” position
(4) Located as specified in 424.19

So, you can use a thermostat as both a controller and the additional disconnect.
Now, elsewhere in the Code it is allowed that a switch can be used as a disconnect provided it is sized for the load it serves.

404.14 Rating and Use of Switches. Switches shall be used within their ratings and as indicated in 404.14(A) through (F).
*Informational Note No. 1: For switches on signs and outline lighting, see 600.6.
*Informational Note No. 2: For switches controlling motors, see 430.83, 430.109, and 430.110.
(A) Alternating-Current General-Use Snap Switch.  A form of general-use snap switch suitable only for use on ac circuits for controlling the following:
(1) Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage applied

So, the branch circuit must be sized 125% of the load but the switch or thermostat in this case only has to be sized for the load. Consequently, this is why all the thermostats you have found are only rated for 16 amps since most branch circuits supplying these would be 20 amps and that would limit the load to 16 amps.