[mxplxxx (OP)]

A frame of reference, accordingly to relativity, is a collection of particles, bound together by gravity, having the same momentum (I think!). Given that momentum seemingly has an infinite number of values, what are the chances of two particles being close together with the same momentum?  In other words what are the chances of frames of reference forming? Vanishingly small I would think. This makes sense given that everything in the universe is constantly being accelerated (e.g. world spins, revolves around the sun, solar system spins etc.). In other words, it would appear there cannot exist anywhere in the universe a frame of reference which, by definition, has constant velocity.

[PmbPhy]

A frame of reference, accordingly to relativity, is a collection of particles, bound together by gravity, having the same momentum (I think!).

There's no such thing as Einstein’s frame of reference so I'm going to assume that you're just talking about a frame of reference itself. A frame of reference is correctly defined here:
http://en.wikipedia.org/wiki/Frame_of_reference

In physics, a frame of reference (or reference frame) may refer to a coordinate system used to represent and measure properties of objects, such as their position and orientation, at different moments of time. It may also refer to a set of axes used for such representation. In a weaker sense, a reference frame does not specify coordinates, but only defines the same 3-dimensional space for all moments of time such that the frame can distinguish objects at rest from those that are moving.

In Einsteinian relativity, reference frames are used to specify the relationship between a moving observer and the phenomenon or phenomena under observation. In this context, the phrase often becomes "observational frame of reference" (or "observational reference frame"), which implies that the observer is at rest in the frame, although not necessarily located at its origin. A relativistic reference frame includes (or implies) the coordinate time, which does not correspond across different frames moving relatively each other. The situation thus differs from Galilean relativity, where all possible coordinate times are essentially equivalent.

Such things do exist.

Given that momentum seemingly has an infinite number of values, what are the chances of two particles being close together with the same momentum?  In other words what are the chances of frames of reference forming? Vanishingly small I would think. This makes sense given that everything in the universe is constantly being accelerated (e.g. world spins, revolves around the sun, solar system spins etc.). In other words, it would appear there cannot exist anywhere in the universe a frame of reference which, by definition, has constant velocity.

You're making the mistake that a lot of amateurs do, i.e. they confuse what really exists in nature with how we describe them. For instance; suppose I describe a the object known as a "basketball" (as opposed to the game) and describe it as given on Wiki, i.e.
http://en.wikipedia.org/wiki/Basketball_(ball)

A basketball is a spherical inflated ball used in a game of basketball.

Since there are no physical objects which have that exact same shape but only approximately does one could argue that basketballs really don't exist. This is the kind of argument that you just gave and its wrong for the same reasons that the argument I just gave that basketballs don't exist. I.e. never expect a description to be perfect but only to be a model of what you're describing.

[jeffreyH]

Frames of reference start in classical mechanics and are coordinate systems with a time element that do not vary as they do in relativity. This is because at non-relativistic velocities they describe nature to an acceptable accuracy.